Preheat oven to 350°F
Heat butter in skillet on medium heat
Chop onion
Saute onion, mushroom, and butter in preheated skillet until onion is transparent
Add spinach and lemon juice
Mix well
Spray casserole dish with cooking spray
Cover with 1/2 cup of the alfredo sauce
Sprinkle with half of the cheese
Place tilapia on top of mixture
Cover with remaining alfredo sauce
Sprinkle with remaining cheese
Bake for 20 minutes in preheated oven
Serve