Preheat the oven to 375°F (190°C). Grease a 12-cup muffin tin (or two) with cooking spray.

On a lightly floured surface, unroll the thawed pie crusts. Using a 3-inch round cutter, cut out 24-30 circles. Re-roll scraps to get more circles.

Gently press each pastry circle into the muffin cups. Prick the bottoms with a fork and chill the crusts in the refrigerator for 15-20 minutes.

In a large bowl, whisk together 6 large eggs until combined. Add 1 ½ cups half-and-half and whisk gently.

Stir in ½ teaspoon salt, ¼ teaspoon black pepper, and a pinch of freshly grated nutmeg. Whisk well.

Gently fold in ½ cup shredded Gruyère cheese, ½ cup crumbled cooked beef bacon, ¼ cup squeezed dry spinach, and 2 tablespoons chives (if using).

Remove the chilled crusts. Stir the filling, then carefully ladle it into each pastry cup, filling two-thirds to three-quarters full.

Sprinkle the reserved ¼ cup Gruyère cheese over each filled quiche.

Bake in the preheated oven for 25-30 minutes, until golden brown, lightly puffed, and set with a slight wobble in the center.

Let quiches cool in the muffin tins for 5-10 minutes, then transfer to a wire rack to cool completely. Serve warm or at room temperature.