Pulse flour, baking powder, and salt in a food processor to combine. Add butter and pulse until pea-sized pieces form. Add water; pulse briefly. Transfer dough to a floured work surface; form into a disk. Wrap in plastic wrap; chill for 1 hour.
Heat oven to 425°F Transfer dough to a floured work surface. Roll dough into a 13-inch circle; transfer to an 11-inch fluted tart pan with a removable bottom, pressing into bottom and sides. Trim excess dough; chill for 30 minutes. Prick bottom of dough with a fork; cover bottom with a sheet of parchment paper; fill with pie weights or dried beans. Bake until crust is set, about 20 minutes. Remove paper and b eans; bake until crust is light brown, about 15 minutes. Let cool.
Cook bacon in a 4-quart saucepan over medium heat until crisp, 7-8 minutes. Using a slotted spoon, transfer bacon to a plate. Discard bacon fat; add butter. Add onions and season with salt; reduce heat to low, cover, and cook, stirring occasionally, until soft, about 10 minutes. Uncover; cook until lightly browned, 10 minutes. Let cool.
Whisk milk, cream, and eggs in a bowl; add reserved bacon and onions along with cheese. Season with salt, pepper, and nutmeg. Pour filling into tart; place on baking sheet.
Place tart in oven and reduce heat to 400°F Bake until filling is golden brown and set, 45-50 minutes. Transfer tart to a wire rack to let cool completely before slicing and serving.