Make this dish ahead. It keeps in the fridge (covered) for a week. Served alongside a crisp salad, it makes the perfect lunch or light dinner.
Substitution(s): Use sharp cheddar instead of mozzarella for a tangier bite!
Preheat oven to 375° F. In a large non-stick skillet cook sausage over medium-high heat until evenly browned, about 8 minutes. Remove sausage from pan and place on a paper towel lined plate to drain.
Wipe out skillet with a paper towel. Place olive oil, squash and onions in skillet and cook over medium heat until tender, about 6 minutes. Set aside to cool for a few minutes.
Place eggs, cheese, milk, salt, pepper and parsley into a large bowl and whisk to combine. Stir in sausage, squash and onions. Place empty pie shell onto a baking sheet and pour mixture into it. Place in oven and bake for 40 to 45 minutes or until pie is set and slightly golden brown. Let stand for 5 minutes before cutting.