Preheat oven to 350F. In a large bowl, whisk eggs with sour cream and half the cheese. Set aside.
Heat a 10-inch ovenproof skillet over medium-high heat. Add 1 tbsp of olive oil and sliced mushrooms, season with ¼ tsp salt. Cook until liquid releases and mushrooms are golden, about 5 minutes. Remove mushrooms and set aside.
Return pan to heat, add remaining olive oil and cherry tomatoes. Cook, stirring occasionally for another 4 minutes. Tomatoes should start to blister.
Add mushrooms back to pan, pour egg mixture on top, sprinkle with remaining cheese, green onion and parsley. Season with black pepper. Allow to cook on stovetop until edges have set, about 4-5 minutes. The egg mixture will still be runny.
Bake in the oven for 20 minutes or until eggs have puffed up. Cool for 10 minutes, run a silicone spatula around the edge of the pan and underneath the frittata, it should slide right off onto your serving dish. Slice into 8 pieces and serve.