Heat olive oil in a large skillet over medium-high heat. Add mushrooms, onion and garlic and saute for 5-7 minutes until tender.
In a large mixing bowl, combine mushroom mixture with cream cheese, mayonnaise, ¼ cup Pecorino Romano cheese, ¼ cup Parmigiano Reggiano, parsley, black pepper and cayenne pepper. Stir until all ingredients are well blended.
Spray 2-quart casserole dish with cooking spray and then add in dip. Ensure it has been evenly distributed in the casserole. Top with remaining grated cheese.
Bake for 30-40 minutes until top is lightly browned.
Notes
To make a lightened up version of this recipe, replace regular cream cheese with reduced-fat cream cheese and regular mayonnaise with light mayonnaise.*Note that traditional Pecorino Romano and Parmigiano Reggiano are not technically vegetarian as they contain animal rennet. However, BelGioioso and 365 by Whole Foods are two brands that offer a vegetarian-friendly Parmesan. Feel free to replace both the Pecorino Romano and Parmigiano Reggiano with a vegetarian-friendly Parmesan if desired.