This bistro classic is as refined as it is rich. We kept the filling onion-forward by cooking the strands in rendered bacon fat and stirring them into a light custard. Use yellow or white onions here; sweet onions will make the tart too sweet. Use a 9-inch tinned-steel tart pan.
FRENCH ONION AND BACON TART
Start to finish: 2 hours, 15 minutes
1 1/4 cups (6 1/4 ounces) all-purpose flour
1 tablespoon sugar
1/2 teaspoon salt
8 tablespoons unsalted butter, cut into 1/2 inch cubes and chilled
2-3 tablespoons ice water
4 slices bacon, cut into 1/4 inch pieces
1 1/2 pounds onions, halved through root end and cut crosswise into 1/4 inch slices
3/4 teaspoon salt
1 sprig fresh thyme
2 large eggs
1/2 cup half-and-half
1/4 teaspoon pepper
Spray 9 inch tart pan with removable bottom with vegetable oil spray. Pulse flour, sugar, and salt in food processor until combined, about 4 pulses. Scatter butter over top and pulse until mixture resembles coarse sand, about 15 pulses. Add 2 tablespoons ice water and process until clumps form and no powdery bits remain, about 5 seconds, adding up to 1 tablespoon more ice water if dough will not form clumps.
Tear dough into walnut-size pieces and sprinkle evenly in pan. Working outward from center, press dough into even layer, sealing any cracks. Working around edge, press dough firmly into corners, up sides of pan, and into fluted ridges. Use your thumb to level off top edge of dough. Use excess dough to patch any holes. Lay plastic wrap over dough and smooth out any bumps or shallow areas. Place dough-lined pan on plate and freeze for 30 minutes.
Adjust oven rack to middle position and heat oven to 375 F. Place frozen tart shell on baking sheet. Spray piece of extra-wide heavy-duty aluminum foil with vegetable oil spray and gently press against dough, covering edges to prevent burning. Fill with pie weights and bake until top edge of dough just starts to color and surface no longer looks wet, about 30 minutes. Remove foil and weights. Return sheet to oven and continue to bake until tart shell is golden brown, 5 to 10 minutes longer. Set sheet with tart shell on wire rack. Do not turn off oven.
Meanwhile, cook bacon in 12 inch nonstick skillet over medium heat until crispy, 5 to 7 minutes. Using slotted spoon, transfer bacon to paper towel-lined plate. Pour off all but 2 tablespoons fat from skillet (if you do not have 2 tablespoons fat, add vegetable oil as needed to make this amount).
Add onions, salt, and thyme sprig to skillet. Cover and cook until onions release liquid and start to wilt, about 10 minutes. Reduce heat to low and continue to cook, covered, until onions are very soft, about 20 minutes longer, stirring once or twice (if after 15 minutes onions look wet, uncover and continue to cook 5 minutes longer). Remove pan from heat and let onions cool for 5 minutes.
Whisk eggs, half-and-half, and pepper together in large bowl. Discard thyme sprig. Stir onions into egg mixture until just incorporated. Spread onion mixture over tart shell and sprinkle bacon evenly over top. Bake tart on sheet until center feels firm to touch, 20 to 25 minutes, rotating pan halfway through baking.
Transfer sheet to wire rack and let tart cool for at least 10 minutes. Remove outer metal ring of tart pan, slide thin metal spatula between tart and pan bottom, and carefully slide tart onto platter. Serve warm or at room temperature.
To make ahead: Cooled tart can be refrigerated wrapped in plastic wrap for up to 3 days. Reheat on baking sheet in 325 F oven for 10 to 15 minutes.