Preheat the grill or oven to 400°F.
Lightly grease an 8x8-inch baking dish or cast-iron skillet.
Squeeze as much moisture as possible from the spinach and place it in a mixing bowl.
Crumble the feta cheese using your hands or a fork, then add it to the bowl.
Add the remaining ingredients, reserving 1/2 cup of the mozzarella.
Stir until everything is well combined.
Spread the mixture evenly into the prepared dish and sprinkle the remaining mozzarella over the top.
Bake for 25-30 minutes, or until hot, bubbly, and golden around the edges.
Serve warm with your favorite chips or crackers.