Preheat oven to 375° F.
Heat a 10” cast iron skillet over medium heat for a few minutes, then add the olive oil.
Once the oil is hot, add the onions and garlic. Cook over medium heat, stirring occasionally to avoid burning. Onions should begin to caramelize after about 10 minutes.
Add the zucchini, squash and salt and stir. Continue to cook for 8 additional minutes, stirring occasionally to avoid burning.
Add the mushrooms and cook for 5 minutes.
Add the smoked salmon and gently stir to combine. Cook for 5 minutes.
In a separate bowl whisk together the eggs, milk, ricotta and feta.
Even out the top of the veggie mixture as much as possible with a spatula. Pour the egg mixture over the veggies in the cast iron. Do not stir. Cook on the stove for 3 minutes.
Transfer the cast iron skillet into the oven and cook for 13 minutes.
Check to make sure that the top is starting to set. It can still be a little liquidy on top, but shouldn’t look totally soupy.
Switch the heat to a broil, and broil for 3 minutes.
Remove frittata from oven and let sit for 5 minutes before serving.
Slice and enjoy!