Preheat your oven to 350°F (175°C) and coat a 7 ½"x10" baking dish with nonstick spray.

Microwave the broccoli florets covered with a damp paper towel for 4-5 minutes until tender.

In a mixing bowl, combine the cooked broccoli with softened cream cheese, mayonnaise, minced garlic, onion powder, salt, and black pepper. Stir in the chopped spinach and half of the cheddar cheese.

Spread the mixture evenly in the prepared baking dish and sprinkle the remaining cheddar and mozzarella cheese on top.

Bake in the preheated oven for 30-35 minutes until the top is golden brown and bubbly.

Let the casserole rest for a few minutes before serving.