Preheat oven to 400 F. Slice peppers in half and scoop out seeds/white piths. Drizzle peppers with a bit of olive oil and season with salt & pepper. Bake 10 minutes, then remove from oven.
Meanwhile, heat a large skillet over med-high heat. Add ground beef, breaking up with a spoon.
Once beef is browned, add onion and garlic, sauteeing for another 5 minutes. Stir in pasta sauce and salt & pepper, bringing to a boil. Stir in spinach, then remove from heat.
Add beef mixture to each pepper, filling up about halfway. Spread with a layer of ricotta cheese (about 2 tbsp), then finish with more beef mixture. Top with mozzarella cheese.
Bake peppers for another 12-15 minutes until cheese is melted. Remove from oven, garnish with fresh herbs if desired, then serve and enjoy!
NOTE: One serving is half a pepper and is noted in the nutritional values. One half may not seem like a lot but it's filled with 28g of protein! I suggest serving with a side salad for a complete meal.