Heat a skillet over medium and add olive oil. Add squash and sauté for 6-8 minutes, until fork tender. Remove from pan with a slotted spoon.
Add leeks to pan and sauté for 3-4 minutes, until softened. Add in garlic and continue cooking for 1-2 minutes. Remove from heat.
Add eggs, milk, salt and pepper to a large mixing bowl and whisk to combine.
Stir in feta and cooked squash/leek and garlic mixture.
Pour the egg mixture into the pie crust. Bake for 35-40 minutes, or until the eggs are set.