Preheat your oven to 350°F.
Roll out the pie dough on a floured surface until it’s about 12 inches in diameter. Transfer to a 9-inch deep pie dish and press into the bottom and sides. Trim excess dough, leaving about 1/2 inch overhanging, then crimp the edges.
Prick the bottom of the dough with a fork and place the crust on a small sheet pan. Par-bake for 10 to 15 minutes until lightly golden, then set aside.
Meanwhile, heat olive oil in a skillet over medium heat. Sauté the red onion and garlic for 30 seconds until fragrant. Add zucchini, 1/2 teaspoon salt, and pepper to taste. Cook, stirring occasionally, for 7-8 minutes until softened.
In a bowl, whisk together the eggs, milk, ricotta, mozzarella, Parmesan, basil, and the remaining salt and pepper.
Fill the par-baked crust with three-quarters of the sautéed zucchini, then pour in the egg mixture. Top with the remaining zucchini and a sprinkle of Parmesan.
Cover the crust edges with aluminum foil to prevent overbrowning. Bake for 50–60 minutes, or until the quiche is set and the top is golden brown.
Allow the quiche to cool for 10 minutes before slicing. Garnish with fresh basil and serve.