Thaw the spinach in the microwave and squeeze it well, then transfer to a medium-sized bowl.
Drain the artichoke hearts and add them to the bowl with the spinach along with the sour cream, yogurt, garlic, salt, pepper, onion powder and lemon juice.
Puree everything well with a hand blender, then fold in the parmesan and mozzarella.
Season with more salt and pepper, depending on your taste.