Preheat oven to 350 degrees

In a mixing bowl, whisk together eggs, milk, 3/4 teaspoon salt and 1/2 teaspoon pepper until just combined.

Heat a 12-inch cast iron (or other ovenproof skillet) skillet to medium heat. Add olive oil to coat pan. Saute onion for 3-4 minutes, until tender. Add garlic and zucchini and toss to cook until tender, about 3-5 minutes. Push everything to the side of the pan and add tomatoes, tossing to cook until lightly blistered and starting to burst. Sprinkle everything lightly with salt and pepper.

Turn pan heat to low. Add butter to pan and toss everything together, arranging vegetables in an even layer. Sprinkle cheese and basil on top.

Gently pour egg mixture over the vegetables and place pan in oven. Cook for about 15 minutes, or until eggs are set. Let cool for a few minutes and then slice into wedges. Serve with pesto.