Preheat your oven to 325°F (165°C). If using a glass pie plate, preheat to 300°F (150°C). Generously butter a 9-inch pie plate and set aside. 2.
In a large bowl, whisk together the eggs, milk, grated Parmesan cheese, salt, and a generous amount of freshly ground black pepper until smooth and slightly frothy. 3.
Heat the olive oil in a large skillet over medium heat.
Add the diced onions and sauté for about 5 minutes until softened and translucent. 4.
Stir in the well-drained tomatoes and basil, and continue to cook for 10 minutes, stirring occasionally, to allow the flavors to meld and excess moisture to evaporate. 5.
Add the sliced zucchini to the skillet and cook for another 5 minutes, until it's tender-crisp. 6. Carefully pour the hot vegetable mixture into the bowl with the whisked eggs and stir to combine everything evenly.
Pour the entire mixture into your prepared pie plate. 7.
Bake for 15-17 minutes, or until the edges are set and the center is just barely wobbly. A knife inserted near the center should come out mostly clean.
Let it rest for 5 minutes before cutting into wedges and serving warm.
Don't skip draining the tomatoes thoroughly. Excess liquid can make your frittata watery and prevent it from setting properly.
