Preheat oven to 425 degrees F. Spray a baking dish with nonstick cooking spray.
Add the onion to a nonstick skillet sprayed with cooking spray. Let the onions cook for 5 minutes. Add the minced garlic during the last minute.
While the onions cook cut the peppers in half and remove the seeds and white parts. Add the peppers to the baking dish.
Add the thawed and drained spinach to the onions. Stir to combine and let cook for 1 minute.
Remove the skillet from the heat and stir in the ricotta and mozzarella cheese, salt, and pepper.
Stuff the peppers with the spinach and ricotta mixture.
Bake for 25 minutes or until the cheese mixture is heated through and the top of the peppers are slightly golden.