It never wins prizes for its looks, but this rustic pastry with caramelized onions, Gruyere, and bacon always wins hearts. When I serve it as a main course, I usually make one large tart, but since this was to be an app, I made two narrower tarts that could be cut into equal sections. Beware: there might be butter.
Tarte à L'oignon—Rustic Onion Tart
2-3/4 cups flour
1/2 tsp salt
1/2 pound butter, chilled and cut into cubes
1 tsp cider vinegar
1/2 to 3/4 cup ice water
In the bowl of a food processor fitted with the steel blade, pulse the flour and salt briefly to combine, then add the butter and pulse several times until it resembles very coarse meal.
In a small bowl, beat the egg with the vinegar and 6 Tbs of water. Drizzle this over the flour mixture and pulse again. Add water, 1 Tbs at a time, pulsing until the dough starts to clump; don't let it come together into a ball.
Turn the dough out onto a floured surface and knead it together deftly into ball, taking care not to overwork it. Flatten slightly into a disk, wrap in plastic, and refrigerate.
1 Tbs butter
1 Tbs flour
1 cup milk
1/4 tsp salt
4 slices thick-cut bacon
3 medium or 4 small onions, coarsely diced
1/3 pound Gruyere cheese, grated
1 Tbs chopped fresh thyme
Cracked black pepper
In a small saucepan, melt the butter and add the flour. Cook, whisking, 2 to 3 minutes. Add the milk gradually and cook, whisking, until it comes to a boil. Whisk in the salt and remove from the heat to cool.
In a large skillet, cook the bacon until it's crisp, then remove to paper towels to drain. Pour off all but 1 Tbs of fat from the pan and return it to the heat. Add the onions and cook, stirring occasionally, until golden and just beginning to caramelize, about 10 minutes. Add the thyme and cook another minute more. Remove from the heat.
Preheat oven to 375ºF.
Remove the dough from the fridge and ready a baking sheet no larger than 12 x 16 inches. If you're making one large tart, roll the dough into a rectangle a little larger than the sheet. Carefully lay the pastry on the sheet. If you're making two long tarts, divide the dough and roll it into two narrow rectangles about 8 x 14 inches; lay these crosswise on the sheet.
Spread the dough with the white sauce, leaving a 1 or 2 inch margin for the crust. Layer the onions onto the sauce, then crumble the bacon and scatter it over the onions. Top with the grated cheese and cracked black pepper. Fold the edges of the dough over the filling, making a 1 to 2 inch border of crust.
Bake 35 to 40 minutes, until the pastry is golden and the filling is bubbling and browned. Cool briefly, then cut into sections and serve.