Preheat oven to 400
Heat a medium cast iron or oven-safe skillet over medium heat. Add the olive oil and onions to the pan. Stir the onions and cook until translucent.
Add the zucchini and capers and stir until the zucchini start to brown. Once the zucchini are brown, add the salt and stir.
While the zucchini is cooking whisk the eggs, milk, and 1/4 cup of ricotta in a large bowl.
Pour half of the egg mixture to the pan. Spoon a few dollops of marinara or tomato sauce over the top. Add the rest of the egg mixture to the pan and spoon the rest of the marinara over the top. Dollop the rest of the ricotta across the top. Don't worry about making the marinara or ricotta perfectly even, you want pockets of these flavors throughout the frittata.
Let the frittata cook on the stove top for 3-4 minutes or until the edges start to firm up just a bit. Place the pan in the oven and bake for 15-20 minutes or until the top of your frittata has puffed up and turned golden.
Remove from the oven and allow to cool 10 minutes before slicing and serving.