Place a rack in the upper third of oven. Preheat oven to 350 degrees F.
Heat 1 Tbsp oil in oven-safe pan over medium heat. Add leeks and cook until softened. Stir often. Add in mushrooms. Continue to stir and cook until mushrooms have given off most of their moisture and have begun to brown.
Mix together mozzarella and fontina cheese. Reserve a little over 1/4 cup of cheese.
Whisk together eggs along with sour cream, thyme and 3/4 cup mixed cheese. Season with salt and pepper.
Once leeks and mushrooms are done, add remaining 1 Tbsp oil to the pan. Pour in whisked egg mixture. Quickly and gently swirl leeks and mushrooms around the pan to ensure the eggs are evenly distributed. Increase heat to medium-high and cook without stirring until the edges of the frittata begin to set.
Sprinkle reserved cheese over the eggs. Bake in preheated oven for 15-20 minutes until frittata the center is set and golden brown. Garnish with more cheese and thyme as desired. Serve warm and enjoy with toast.