Preheat the oven to 425 degrees F. Line a baking sheet with parchment paper.
On the prepared pan, toss the zucchini, summer squash and cherry tomatoes with 2 tablespoons olive oil and 1 teaspoon salt. Arrange in an even layer. It may seem like a lot of veggies, but they will shrink down.
Bake the zucchini and squash for about 20 minutes, until soft, lightly browned and caramelized. Toss to redistribute once, then return to the oven for 5 more minutes. Remove from the oven and set aside. The vegetables can be make up to 3 days in advance.
Meanwhile, in a large mixing bowl, beat the eggs with 1/2 teaspoon salt until the yolk and whites are very well combined.
In a large (15-inch) oven-proof skillet, heat 1 tablespoon of the oil over a medium-low flame and swirl to coat the pan. Add half the squash mixture in an even layer. Pour the eggs over the vegetables, making sure the pan is evenly coated with eggs and scatter with half the basil. Cook until the sides are set and there’s just a shallow layer of uncooked eggs on the top, about 5 minutes.
Sprinkle the remaining zucchini mixture and feta (if using) over the top and transfer the pan to the oven. Cook for 2 to 5 minutes, until the rest of the egg is set. Alternatively, you can turn on the broiler for 1-2 minutes.
Allow the frittata to sit in the pan for at least 5 minutes before slicing. Garnish with the remaining basil, cut into wedges and serve warm alongside a simple salad as a low FODMAP breakfast, lunch, or dinner.