In a large skillet heat 1 Tbls olive oil over medium high heat. Add mushrooms, and onions , and cook for 5-6 minutes until lightly browned. Mix in garlic, and cook for 1 minute until fragrant. Remove from pan.
Return the skillet to heat and brown sausage until fully cooked. Drain and mix with mushroom mixture. Season with salt and pepper to taste.
Mix in 1 cup of Mozzarella cheese, and pour into a casserole dish. Sprinkle remaining cheese over the top.
In a bowl mix together breadcrumbs and remaining 3 Tbls olive oil. Spread evenly over the top of the casserole.
Bake for 15 minutes until hot, cheese is melted, and breadcrumbs are golden brown. Remove and serve immediately.