Melt the butter over medium-high heat in a 10-inch, heavy, oven-proof skillet, preferably well-seasoned cast iron.
Add the onions and smashed clove of garlic and sauté until softened and the garlic is fragrant, about 2 minutes (meanwhile, prep the other veggies).
Add the zucchini, summer squash, and cherry tomatoes, along with a generous pinch of kosher salt and black pepper. Sauté, stirring occasionally but not too frequently, until veggies are softened and there is almost no liquid left at the bottom of the skillet (5-10 minutes).
Meanwhile, beat the six eggs with the 3/4 cup milk and season with a pinch of salt and pepper.
Once the veggies are cooked, pour the egg and milk mixture into the skillet and add the 1 cup of mozzarella cheese. Stir around to distribute the cheese. Allow the eggs to cook for about two minutes on the stovetop, scooting them from the outsides of the skillet into the center as they cook, until the eggs start to look more solid throughout but definitely aren't all the way cooked yet.
Move the skillet to the oven and broil for 5-10 minutes, or until eggs are fully cooked and starting to brown slightly on top (if you shake the pan, they don't jiggle).
Allow to cool for at least 5 minutes, then slice and serve sprinkled with fresh basil and some extra salt and pepper, if desired.
Add bacon to make it more hearty: Cook up 4 strips of bacon in the skillet, remove and chop, and proceed with cooking, using two tablespoons of the bacon fat instead of the butter to cook the onions. Add the chopped bacon back into the skillet when you add the egg mixture.
Money saving tip: Summer produce is cheaper in (you guessed it!) the summer, so if you make this off-season it may be more expensive (and less tasty).
Time saving tip: You can cook the veggies in advance. Roast them, sauté them, or grill them up to 2 days in advance, and simply reheat in the butter before adding the egg mixture and cheese.
For picky eaters: I recommend grating the zucchini and summer squash, and omitting the tomatoes, for picky kids who "don't like veggies." You won't even notice they're there!
Make it paleo/whole30: use olive oil instead of butter, omit the cheese and milk, and add 2-4 more eggs.
The provided nutrition information does not include any added sodium from seasoning to taste, any optional ingredients, and it does not take brands into account. Feel free to calculate it yourself using this calculator or by adding the recipe to Yummly.