Preheat oven to 450°. Bring a medium pot of water to a boil. Pour oil into a 12- by 17-in. rimmed baking pan and lightly oil with a paper towel. Sprinkle pan with cornmeal.
On a lightly floured surface, stretch dough with hands and/or roll with a rolling pin into a rectangle roughly the same size as the pan. Transfer dough to pan, stretching to fit. Fold over 1/2 in. of dough around edges to form a slightly raised rim for the tart. Prick dough all over with a fork.
Add bacon to boiling water and blanch 30 seconds. With a slotted spoon, transfer to paper towels to drain. Add onions to water and blanch 2 minutes, then drain in a colander and spread evenly over dough. Scatter cheese over onions, then sprinkle bacon over cheese. Sprinkle with salt and pepper.
Bake tart until cheese is melted and beginning to brown and crust is crisp and well browned, 20 to 25 minutes. Lift out of pan with a spatula, cut into roughly 2-in. squares, and serve immediately.
Make ahead: The tart can be made up to 2 days ahead and kept wrapped in waxed paper at room temperature. To serve, unwrap and reheat 10 minutes at 200°.
Note: Nutritional analysis is per hors d'eouvre.