Preheat oven to 400°F. Spray a 2-quart baking dish with cooking spray and set aside.
In a large skillet, brown sausage, mushrooms, and onion over medium-high heat until no longer pink. Drain off excess grease.
In a medium bowl, combine cream cheese, Parmesan, and 1 cup mozzarella. Spread ½ of the cream cheese mixture onto the bottom of the prepared baking dish.
Top first layer of cream cheese with ½ of the marinara sauce.
Top the marinara with ½ of the sausage mixture. Sprinkle ½ of the pepperoni on top.
Repeat layers.
Top with an additional 1 cup of mozzarella and additional pepperoni, if desired.
Bake for 20-30 minutes or until cheese is melted and bubbly.
Notes
For convenience, look for bulk Italian sausage. This way you don't have to remove the casings from the sausage on your own.
For another shortcut, look for a container of sliced mushrooms and use frozen pre-diced onion. You won't even have to pull out your cutting board!
I prefer mini pepperoni, since I don't have to chop it before adding it to the dip. You can substitute with chopped pepperoni or turkey pepperoni as well.
Bake the dipon top of a baking sheet in case any bubbles over. This makes cleanup easy.
Use full-fat block-style cream cheese for the best texture.