1stick8 Tablespoons, 4 ounces unsalted butter, cut into ½-inch cubes and well chilled
2 to 3Tablespoonsice water
5slicesbaconcut into ¼-inch pieces
1 1/2 to 1 3/4pounds brown onions (about 3 large) cut crosswise into ¼-inch slices
4 sprig fresh thyme
1 teaspoontable salt
¼teaspoonfreshly ground black pepper
1 heaping tbs dijon mustard
Spray 9-inch tart pan with nonstick cooking spray; line bottom with parchment paper. Combine flour, sugar, and salt in food processor with 5 1-second pulses. Scatter butter pieces over flour mixture and pulse to cut butter into flour until mixture resembles coarse sand, about fifteen 1-second pulses. Add 2 Tablespoons ice water and process until large clumps form and no powdery bits remain, about 5 seconds, adding up to 1 Tablespoon more water if dough will not form clumps.
Transfer the dough to the greased tart pan and, working outward from center, press dough into even layer, sealing any cracks. Working around edge, press dough firmly into corners of pan with index finger. Go around edge once more, pressing dough up sides of tart pan and into fluted ridges. Use thumb to level off top edge of dough. Lay plastic wrap over dough and smooth out any bumps or shallow areas. Place tart shell on plate and transfer to freezer for 30 minutes.
Adjust oven rack to middle position and heat oven to 375° F. Remove plastic and place frozen tart shell on baking sheet; lightly spray one side of 18-inch square extra-wide heavy-duty aluminum foil with nonstick cooking spray. Press foil greased side down inside tart shell, folding excess foil over edge of pan. Fill with dried beans or pie weights and bake until top edge just starts to color and surface of dough under foil no longer looks wet, about 30 minutes. Remove from oven and carefully remove foil and weights by gathering edges of foil and pulling up and out. Return baking sheet with tart shell to oven and bake until golden brown, 5 to 10 minutes. Set baking sheet with tart shell on wire rack. Cool completely. If cracks form in shell, brush with egg white to seal.
While crust is baking, cook bacon in 12-inch nonstick skillet over medium heat until browned and crisp, 8 to 10 minutes. Use a slotted spoon to transfer bacon to paper towel-lined plate. Leave bacon fat in skillet.
Add onions, salt, and thyme 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 10 minutes, stirring once or twice (onions will look wet, after 10 minutes, remove lid and continue to cook another 15-20 minutes). Remove pan from heat and let onions cool 5 minutes.
Whisk eggs, half-and-half, dijon and pepper together in large bowl. Remove thyme sprig from onions; discard. Using a slotted spoon, scoop onions into egg mixture and mix until just incorporated. Spread onion mixture over baked crust (still on baking sheet) and sprinkle bacon evenly on top.
Bake at 375° F until center of tart feels firm to touch, 25 minutes. Cool on wire rack at least 10 minutes. When ready to serve, remove tart pan ring; gently slide thin-bladed spatula between tart pan bottom and crust to loosen, then slide tart to serving plate. Cut into wedges and serve.