Preheat oven to 375°F | 190°C.Butter a 1½ or 2-quart casserole dish, or spray with non-stick cooking spray.
Melt butter in a medium skillet over medium heat. Add onions and cook, stirring frequently, until the are translucent; about 7-8 minutes. Add chopped garlic and cook 1 minute more. Remove from heat and set aside.
Using a whisk, beat one egg into the cream cheese until relatively smooth. Whisk in the remaining eggs.
Stir feta, parmesan, basil, and pepper into egg mixture.
Fold in zucchini, tomatoes, and sautéed onions and garlic.
Pour into prepared casserole dish and bake in preheated oven at 375°F | 190°C for 1 hour, or until the internal temperature reaches 165°F | 74°C.
Remove from oven, cover, and let stand for at least 10 minutes before serving.
To serve, sprinkle with additional parmesan cheese and freshly ground pepper.For a delicious flavor boost, drizzle with balsamic glaze.After baking, cheesy zucchini casserole can be stored in the refrigerator for up to 4 days. Do not freeze.