Heat oven to 375°F. Lightly butter a small baking dish.
Thaw spinach and squeeze out as much liquid as possible until very dry.
Beat cream cheese in a bowl until smooth. Mix in sour cream and mayonnaise.
Stir in mozzarella, Parmesan, garlic, onion powder, salt, pepper, and red pepper flakes.
Fold in the drained spinach, lemon juice, and water chestnuts if using.
Spread the mixture into the prepared dish. Sprinkle cheddar evenly over the top.
Bake until hot and bubbling with lightly golden spots, 18–22 minutes.
Rest 5 minutes. Serve warm with tortilla chips, crusty bread, or veggies.