Coat bottom of a large cast iron skillet with olive oil.
Add squash, onions, garlic, marjoram, and salt and pepper to taste and sauté over medium heat until onions are translucent and very aromatic and squash are softened but not cook through, about 5-7 minutes.
In a bowl combine eggs, ricotta, and Parmesan and whisk until smooth.
Add egg mixture to squash-onions and stir gently to incorporate mixture.
Cook frittata on stovetop until eggs start to set; transfer skillet to oven.
Bake for 12-15 minutes, or until firm.
Run knife around edge and turn frittata out onto a large serving plate.
Scatter chopped tomatoes over frittata, drizzle with olive oil, if desired, and slice into wedges; serve immediately.