Preheat the oven to 400.
Sauté mushrooms over medium heat until softened, 7-10 minutes. Season with salt and pepper. Spread softened mushrooms on sheet pan and bake in oven until crispy, 10-15 minutes. Transfer to a bowl.
In the same pan as the mushrooms, sauté diced onions with lots of olive oil (around 1/4 cup) until deeply browned and caramelized, 20 minutes. Season with salt and pepper. Transfer to a bowl.
In a clean pan, sauté Italian sausage until crispy, 10 minutes. Transfer to a bowl.
In another pan, sauté minced garlic in olive oil until fragrant, 2 minutes. Add spinach, season with salt and pepper, and sauté until spinach is wilted, 5 minutes. Transfer to a bowl.
Serve and putting out bowls of toppings and the pan of polenta, and let everyone make their bowls as they please.