Pulse spinach in three batches in a food processor until finely chopped. Transfer to a medium bowl. Add ricotta (or cottage cheese), Parmesan, eggs, garlic, salt and pepper; stir to combine.
Coat 8 cups of the muffin pan with cooking spray. Divide the spinach mixture among the 8 cups (they will be very full).
Bake the spinach cakes until set, about 20 minutes. Let stand in the pan for 5 minutes. Loosen the edges with a knife and turn out onto a clean cutting board or large plate. Serve warm, sprinkled with more Parmesan, if desired.