Equipment: 12-cup muffin pan; pie weights or dried beans
Preheat the oven to 375°F. Cut out twelve 6-inch square pieces of aluminum foil, set aside.
Roll out the dough on a lightly floured work surface. Using a small plate or saucer, cut out twelve 5-inch rounds from the dough. Press each dough round into a muffin cup, then use a fork to prick the bottoms. Line each dough-filled muffin cup with a piece of aluminum foil. Fill each cup halfway with pie weights or dried beans.
Bake the lined muffin cups for 15 minutes. Remove from the oven and when cool enough to handle, remove the pie weights and foil.
While the crusts bake, heat the olive oil in a large skillet over medium heat. Add the onions and the bacon and cook, stirring occasionally, until onions are deeply golden and bacon is crisp, about 15 minutes.
Add the spinach and cook, stirring, until wilted, about 3 minutes. Remove from the heat.
Whisk together the eggs, half and half, salt and pepper in a large bowl. Stir in the cheese and the bacon mixture. Divide the egg mixture evenly between the muffin cups.
Bake the quiches until puffed and lightly golden on top; a skewer inserted in the center should come out clean, 20–23 minutes. Remove from the oven and let cool for 5 minutes (the quiches will deflate) before removing the quiches from the pan. Serve warm or at room temperature.
Variation: To make gluten-free frittatas, omit the pie dough and grease the muffin cups well (the recipe yields 10 to 12 frittatas). Divide the egg mixture evenly between the muffin cups. Bake in a preheated 375°F oven until puffed and lightly golden on top, about 20 minutes. Remove from the oven and let cool for 5 minutes. Run a knife around the edges of the frittatas to loosen, then lift out and transfer to a serving plate.