Preheat oven to 425F
Heat the 2 tablespoons of olive oil in a 10" skillet that can go in the oven. We used a non-stick skillet rated for up to 450F. Add the yellow onion and saute just until it begins to brown and caramelize. Add the sliced squash on top of the onion and continue to cook the vegetables over medium-high heat. Once the squash starts to cook, turn it once. Season the mixture with a little salt and pepper. Don't over season, you'll be adding more salt and pepper to the eggs.
Meanwhile beat the eggs in a small bowl with the cream, fresh herbs, red pepper flakes, and add salt and pepper to taste.
Once the vegetables are cooked tender, top with the eggs and Gruyere cheese. Arrange the thin tomato slices and green onion slices on the top. Cook for about 2 minutes over medium-high heat.
Now place in the hot oven and cook for 10-12 minutes. Every oven is a little different so start checking the fittata after about 8 minutes. Use a think knife and make a little slit in the center of the fittata. If the eggs still run, then bake just a couple minutes more. An over baked frittata will become vulcanized rubber, so beware!
Remove from the oven and serve immediately.