Preheat the oven to 375 degrees F.
Heat the olive oil in a large skillet over medium-high heat. Add the onion and saute, stirring occasionally until translucent, about 5 to 8 minutes.
Add the chopped zucchini, tomato, garlic, and sea salt. Continue sauteing until zucchini has softened but is still al dente, and the skins of the tomatoes begin to wilt, about 5 to 8 minutes.
In a mixing bowl, whisk the eggs until well-beaten.
Transfer the sauteed veggie mixture to an 10-inch round baking pan or casserole dish. Pour in the eggs and use a spoon to evenly spread the veggies and eggs in the pan.
Place on the center rack of the oven with a baking sheet underneath (just in case - to catch any egg that spills over). Bake for 32 to 40 minutes, until eggs have set up and are golden-brown on top. Let frittata sit 15 minutes before slicing and serving.