Preheat the oven to 400°F.
In a 10-inch ovenproof, nonstick skillet, heat the oil, three turns of the pan, over medium-high. Add the garlic and swirl the pan until fragrant, about 1 minute. Add the drained spinach, separating it with your fingers; season with salt, pepper and nutmeg.
In a medium bowl, whisk the eggs and mix in the ricotta and Parm to blend well. Pour into the skillet and cook until the edges are set, about 2 minutes. Transfer to the oven and bake until the top is set, about 12 minutes. Top with the mozzarella and place back in the oven until browned and bubbly, 5-7 minutes.
Cut the frittata in wedges and serve with a tomato-onion salad alongside.