Heat butter or olive oil in a skillet over medium heat. Add garlic and sauté for 30 seconds.
Add spinach and cook until wilted (or heated through if frozen). Set aside.
In a mixing bowl, stir together cream cheese, sour cream, and mayonnaise.
Fold in mozzarella, Parmesan, salt, pepper, onion powder, and optional seasonings.
Add spinach mixture and stir until well combined.
Transfer to a baking dish and bake at 375°F for 15–20 minutes, until hot and bubbly.
Serve warm with bread, chips, or fresh veggies.