Preheat your oven to 375°F (190°C). This will ensure your peppers cook evenly and develop a lovely roasted flavor.
Slice the tops off the red peppers and remove the seeds and membranes. Lightly brush the outside with olive oil and place them in an oven-safe baking dish.
In a large skillet, heat 1 tablespoon of olive oil over medium heat. Add the diced yellow onion and sauté until it's soft and translucent, about 5 minutes. Stir in the ground beef, breaking it apart with a wooden spoon, and cook until browned. Drain any excess fat.
To the skillet with the beef, add the diced garlic and cook for an additional 1-2 minutes until fragrant. Then, stir in ½ cup of the pasta sauce and 2 tablespoons of water. Mix well and allow it to simmer for about 5 minutes, letting the flavors meld together.
Remove the skillet from heat and fold in the diced green onion and half of the fresh mozzarella (chopped into small pieces) into the beef mixture. This will add that irresistible cheesy element right into the filling.
Carefully spoon the beef and cheese mixture into each prepared pepper, packing it down gently. Once all the peppers are filled, pour the remaining pasta sauce evenly over the top.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. After 25 minutes, remove the foil, sprinkle the remaining mozzarella on top of each pepper, and return to the oven uncovered for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Once baked, remove the peppers from the oven and let them cool for a few minutes. Serve warm, garnished with extra diced green onions if desired. Enjoy your delicious Cheesy Italian Stuffed Peppers!