Preheat oven to 375° F. Spray a 2-quart casserole dish with non-stick cooking spray.
In a medium skillet over medium-low heat, caramelize onions & garlic.
Using a whisk, beat marscarpone (or cream cheese) into eggs. (It's fine if there are small lumps.)
Stir feta, Parmesan, and garlic salt into egg mixture.
Fold in zucchini, tomatoes, and caramelized onions and garlic.
Pour into prepared casserole dish and bake at 375 for 45 minutes.
Remove from oven and let stand for 10 minutes before serving.