Artichoke Spinach Dip

3 (10 oz) package frozen spinach, thawed and well drained
3 (14 oz) cans artichoke hearts, drained and chopped
1 medium onion, chopped
1 Tbsp. fresh garlic, minced or garlic powder
1 cup mayonnaise
1 cup parmesan cheese
2 cups cheddar cheese, grated
2 cups Monterey jack cheese, grated
2 (8 oz) packages cream cheese
1 can cream of mushroom soup
1 can mushrooms

Lightly grease large baking dish. Mix all ingredients. Place in baking dish and bake at 350 degrees for 35-45 minutes.