If you love the Thai tom yum, it's time you got some Laotian tom zap.

The two soups, like the two cuisines, are kindred spirits that share many traits, as would be expected from the kitchens of neighboring countries. And that's especially true at Sa-Bai-Dee, the Upper Darby BYOB owned by a couple who bridge the border.

Manager Kanokporn "Pang" Ekawipart is from Thailand, and her husband, chef-owner Ketkeo Banthavong, is from central Laos. So not surprisingly, their menu touches the flavors of both.

But there is a special unvarnished edge here to the Laotian flavors, which set off extra sparks. The tom yum, by any measure of local Thai restaurants, delivers a confident sour twang to its vivid lemongrass broth, enriched with golden chicken stock, a firm tingle of red spice that coats your lips, and slices of chicken breast that are delicately tender.

There is rustic magic in the Laotian zap. Banthavong steeps a simple peasant-style broth from water and chunks of beef flank, stewed for hours with galangal, kaffir lime, lemongrass, and fresh green chiles that make each spoonful sparkle with heat.

That the occasional flap of flank gristle might float to the top amid the fork-tender chunks of flesh will not fluster many here. Since moving to the multi-culti hub of Upper Darby from Havertown, where Banthavong owned Sukothai, the chef has been able to focus on cooking for a largely Asian clientele, many of whom pop by after shopping for fish sauce and stinky durian fruit at the popular H-Mart grocery just a few doors down the sidewalk.

It's so rare to find a Thai restaurant around here that doesn't overcompensate with sweetness for a presumed American clientele, I can understand why Sa-Bai-Dee (a common Thai greeting meaning "I'm good") has become a destination for adventurous non-Asians, too.

While we still lack a Thai spot with quite the same authentic punch you'll find in the boroughs of New York (Ayada; Pok-Pok), SBD's Thai fare is as solid as any I've tasted in our area, and virtually on par with South Philly's Circles.

Spice-hunters might be a little let down, as blazing heat is not the kitchen's default setting - unless, of course, you call ahead to order an off-the-menu specialty like jaew, a Thai dried chile dipping sauce served with grilled vegetables and sticky rice.

But I was impressed simply with the vivid freshness of flavors and delicacy of the cooking. The satay chicken was notably tender and completely infused with its marinade of coconut milk, turmeric, and galangal. The bone-in BBQ chicken app reminded me more of a Thai-style jerk, the gingery, lemongrass-rubbed meat roasted to a juicy brown over the charcoal grill.

The pad Thai was excellent, the fettuccine-wide rice noodles wearing a perfect sweet-and-tangy brown shine (with a faint fish sauce undertow) as they tangled with tender shrimp, cilantro, and roasted peanuts. The wider "spicy noodles" tossed with minced chicken were even better, though their oyster sauce and sweet chili-paste sauce could have used an extra squeeze of sriracha heat.

The chu chee duck, boneless, tender, and crisp beneath a pineapple red curry, is perfect for those still seeking a mild introduction to Thai appeal. For that matter, the crisply fried winter shrimp, seasoned with sesame oil and wrapped with a blade of fresh ginger in a greaseless sheet dumpling skin, was universally irresistible. Likewise the house-made sausage links, bursting with lemongrass, galangal, and fresh chile goodness - and a sensation when those hot brown links are eaten with juicy slivers of ginger, roasted peanuts, and a crunchy red onion in a single bite.

The menu calls them "Thai" for wider appeal to new diners, but Pang concedes they are actually Laotian.

When you venture over to the cold salads, though, there is no hiding the bold fish-sauce funk of Banthavong's Laotian soul.

You can get a beginner's taste with the grilled beef and pork salads - similar to Thai versions, but a notch more assertive on the fish sauce twang that splashes the char-grilled meat in a dressing with lime and chile heat. (I preferred the beef, which took on more char from the grill.)

Those more accustomed to the ocean-like oomph of fermented seafood should head straight for som tum Laos, a shredded slaw of green papaya soaked in a cool, river-brown stew of tamarind, crab paste, and anchovies that you eat by pinching up bundles with triangular sheets of cabbage. The unexpected crunch? Those are chunks of sweet, salt-fermented little crabs, added for extra sea-swagger.

Only slightly less intense, but no less distinctive, was the sup normai, a shred of crunchy fresh bamboo shoots tossed with anchovy-lime zing, roasted rice powder, and a Thai herb called bai yanang. Order it with a wicker-basket side of steaming hot sticky rice, and the two together - pinch of hot rice against cool bamboo crunch - is a memorable combination.

A mound of chicken larb was one of just a few mild disappointments, if only because the minced chicken salad, usually electric with juicy flavor, was too dry. The "serpent fish" curried vermicelli was less exotic than I'd expected. It turned out to be tilapia (not snakehead fished from FDR Park, as I'd secretly hoped) and didn't thrill in its thick yellow-curried stew. The garlicky fried spare ribs, intriguingly soured from a marinade in mashed rice, could have been more tender.

But these were small complaints beside Sa-Bai-Dee's flavorful successes. A gingery pork stew with udon-thick noodles called kao piag sen was another rustic hit, especially doctored up with a shake of chile powder and a spoonful of fish sauce-soaked chiles. A steamed tilapia was a picture of whole-fish elegance, the juicy white flesh infused with lemongrass steam, then shined in the sweet-and-citrusy punch of a chile-lime sauce.

With so many bright flavors swirling across the plates, we took advantage of Sa-Bai-Dee's BYO policy to uncork a full arsenal of quenching rieslings and spice-friendly Belgian-style beers.

But ultimately, it was a dessert of warm coconut-milk-soaked sticky rice paired with sesame-dusted ripe mango that tamed my palate, which shifted once again. This time from "zap" to "yum."

Kanokporn "Pang" Ekawipart talks about Sa-Bai-Dee at www.inquirer.com/labanreviews. Craig LaBan hosts an online chat at 2 p.m. Tuesdays at www.inquirer.com/labanchats. EndText

Next Sunday, Craig LaBan reviews Honey's Sit-n-Eat on South Street. Contact him at 215-854-2682 or claban@phillynews.com.