Preheat oven to 350 degrees Fahrenheit.
In a large pan, cook and crumble ground italian sausage on medium high heat.
When Italian sausage is browned, remove from pan and set aside. Do not drain or clean out pan.
Add onions to pan and let soften and brown, about 5 minutes.
Add garlic to pan and cook down until fragrant and lightly browned, another 3-4 minutes on medium-low. Do not let garlic burn.
Add peppers and mushrooms to pan and cook to softened and lightly browned.
Drain off any excess moisture from vegetables (very important!! I often will squeeze them to get any excess water out of the vegetables.)
Prep a 9x9 casserole dish with coconut oil cooking spray (or your favorite way to prep pans - foil, parchment, butter, lard, cooking spray all work just fine.)
Layer 1/2 the sausage crumbles onto the bottom of the pan.
Add a layer of ricotta, parmesan, and mozzarella (use about 1/3 of each, there will be 3 layers of cheese overall.)
Add a layer of 1/2 the vegetables.
Layer some (about 1/2) the pepperonis across the casserole.
Add a layer of diced tomatoes (and the water they are packed in.)
Layer the rest of the meat, then another cheese layer, followed by the rest of the vegetables and tomatoes.
Finish the casserole with a top layer of cheese and pepperonis, garnish with olives.
Cover casserole with foil and bake for 20 minutes, until casserole is hot and bubbly.
Remove foil and cook until cheese is browned, about another 10 minutes.
Serve immediately and enjoy.