Preheat the oven to 375°F.
Roll out the chilled dough on a floured surface to about 1/8-inch thickness.
Use a round cutter or a glass (about 3-4 inches in diameter) to cut circles from the dough. Press each circle into the cavities of a standard muffin tin, trimming any excess dough. Prick the bottoms with a fork.
Bake the crusts for 8 minutes, then set aside.
Heat the olive oil in a skillet over medium heat. Sauté the spinach for 2-3 minutes, until wilted. Add the sun-dried tomatoes, olives and feta cheese and remove from the heat.
In a large bowl, whisk together the eggs, milk, salt, and pepper.
Evenly divide the spinach filling among the pre-baked crusts in the muffin tin.
Follow with the egg mixture into each crust, filling just to the top but not overflowing.
Bake in the preheated oven for 20-25 minutes, or until the filling is set and the tops are lightly golden.
Let the mini quiches cool for 5 minutes in the tin before carefully removing them with a small knife or spatula.
