For the onions:
1 1/2 pounds (3 medium) yellow onions, peeled, halved and sliced into 1/4-inch half-moons
2 fresh bay leaves
2 tablespoons canola oil
2 tablespoons unsalted butter, cut into small cubes
Freshly ground black pepper
1/2 teaspoon chopped fresh thyme
For the Mornay sauce:
2 tablespoons unsalted butter
2 tablespoons flour
1/2 teaspoon salt
1/4 teaspoon freshly ground black pepper
Pinch grated fresh nutmeg
1 cup warm milk
1 cup grated Gruyere cheese
For the tart:
One 9-inch, prebaked tart shell, in its pan
3 ounces very thinly sliced Virginia or Black Forest ham
1. For the onions: Heat a large saute pan over medium heat for several minutes. Spread the onions evenly in the pan, along with the bay leaves, and let them sit untouched for a couple of minutes. Drizzle the oil evenly over them and dot with butter. Do not stir them for several minutes. (The goal is to let some of the onions' moisture evaporate and to get some good color on the onions at the start. Once you stir, water will be released, and the caramelizing will end until that water evaporates.) Stir the onions, using a flat-edged wooden spoon to scrape up the browned bits from the bottom of the pan. Season to taste with salt and pepper. Let the onions cook, stirring often, for 20 to 25 minutes, until they are golden brown and nicely caramelized. Stir in the chopped thyme. Keep the onions warm on low heat if you are making the tart immediately; or cover and refrigerate for up to 3 days.
2. Meanwhile, make the Mornay sauce: Heat the butter in a small saucepan over medium heat until the butter bubbles. Whisk in the flour, salt, pepper, and nutmeg, and cook for 2 or 3 minutes, to create a nutty-brown roux. Slowly whisk in the milk, beating constantly to prevent lumps. Cook the sauce for 2 or 3 minutes, until well thickened and bubbling. Remove from the heat and add the cheese, stirring until it is melted.
3. For the tart: Position a rack in the upper third of the oven and preheat the broiler to high. Line a baking sheet with aluminum foil. Place the prebaked tart shell on the baking sheet. Spread the warm onions evenly in the bottom of the tart shell, cover with the ham slices, and top evenly with the Mornay sauce. Broil for 4 or 5 minutes, until the sauce is bubbling and nicely browned. Serve immediately.
Per serving: 550 calories, 14 grams protein, 42 grams carbohydrates, 10 grams sugar, 37 grams fat, 95 milligrams cholesterol, 670 milligrams sodium, 3 grams dietary fiber.