Preheat your oven to 375°F (190°C).
Slice each bell pepper in half lengthwise and remove seeds and membranes.
In a skillet over medium heat, add olive oil and sauté onions until translucent, followed by garlic until fragrant.
In a large bowl, combine spinach, ricotta, beaten egg, Parmesan, breadcrumbs, Italian seasoning, salt, and black pepper until well mixed.
Spoon the ricotta mixture into each prepared bell pepper half, pressing down gently.
Place stuffed peppers in a baking dish lined with marinara sauce if desired, cover with foil, and bake for about 30 minutes.