Preheat oven to 350 degrees. On a lightly floured work surface, roll out dough to an 11-by-15-inch rectangle. Transfer to a 9-by-13-inch baking pan. Fold edges of dough so sides are about 1 inch high. Prick dough all over with a fork and freeze until firm, 15 minutes. Press a sheet of parchment paper or foil on dough, draping over rim of pan. Fill with pie weights or dried beans. Bake until crust is firm and edges are lightly browned, about 35 minutes. Remove parchment and weights; bake until bottom is dry and light golden, 10 minutes.
Meanwhile, in a large skillet, cook bacon over medium until almost crisp, 10 minutes, flipping once. Drain bacon on paper towels. Add onion to skillet, season with salt and pepper, and cook, stirring often, until golden brown, 10 minutes. Spread mixture evenly in crust. Top with squash, overlapping slices and adding a piece of bacon every few rows.
In a bowl, whisk together eggs, milk, and cream; season with salt and pepper. Pour enough egg mixture over filling to just reach top of crust. Top with sage. Bake until set in center and puffed at edges, 45 minutes. Let cool 15 minutes.
If using store-bought dough, make this in a deep-dish 9-inch pie plate.