Preheat oven to 350°.
Spray or grease a square 8" casserole dish.
Bring a large pot of salted water to a boil.
Add gnocchi - cook for 2-3 minutes - just until they start to float.
Drain water. Return gnocchi to pot.
Add sauce, ricotta, 3/4 cup mozzarella cheese, spinach & approximately 6 oz. mushrooms. (Save a handful of mushrooms for top of casserole) Add a pinch of red pepper flakes if you'd like a little heat. Stir until combined.
Spoon mixture into baking dish.
Sprinkle remaining 1 1/4 shredded cheese evenly over top. Place reserved mushrooms on top of cheese. I also threw a handful of grape tomatoes on top.
Bake for 30 minutes. If you have tomatoes on top, broil at end of baking time for a minute or two.
Let dish rest for about 10 minutes before cutting. Serve & enjoy :)