Combine 1 cup of mayonnaise, 10 ounces of thawed and drained frozen spinach, a finely minced garlic clove and half a minced onion in a pie plate. Add 1/2 cup of Parmesan cheese and 1 cup of cheddar cheese, and bake at 350 degrees Fahrenheit for 25 minutes.
To create another warm spinach dip, beat 8 ounces of softened cream cheese, 1/4 cup of grated Parmesan cheese and 1/2 cup of mayonnaise until they are well-blended. Stir in 10 ounces of thawed and drained frozen spinach and 4 ounces of shredded mozzarella cheese, and place the mixture in an ungreased microwave-safe dish. Microwave the mixture on high for approximately four or five minutes, stirring at least twice. Serve the dip with crackers or vegetables.