Drain defrosted spinach. Place in a colander and run water through the spinach making sure all ice crystals are gone. Transfer to a dish towel and rig dry.
Coat a casserole dish with cooking spray and pour half a jar of sauce into the bottom. Spread evenly.
Top sauce with an even layer of frozen ravioli. Layer half of the spinach on top and sprinkle with parmesan cheese.
Pour additional sauce and repeat the layering process.
Add a final layer of sauce to the top.
Top with sliced mozzarella.
Bake in preheated oven for 30-40 minutes until sauce is bubbly and cheese is lightly browned.
You can prepare this dish ahead of time and store casserole covered in the refrigerator. Bring to room temperature for about 30 minutes prior to baking. Store covered in the refrigerator for up to 4 days.