Preheat your oven to 375°F (190°C). Slice the bell peppers in half lengthwise, discarding the seeds and membranes. Place them cut-side up on a baking sheet and drizzle with olive oil. Roast for about 20 minutes or until they begin to soften.
While the peppers are roasting, heat a large skillet over medium heat. Add the ground beef, chopped onion, and garlic. Cook until the beef is nicely browned and the onion is translucent, about 7 to 8 minutes. Drain any excess grease.
To the skillet, add the pasta sauce, salt, black pepper, and crushed red pepper flakes for an added kick if desired. Stir to combine and let it simmer for 5 minutes, allowing the flavors to meld.
In a separate bowl, combine ricotta cheese with Italian seasoning and dried oregano. Stir until well blended.
Once the peppers are roasted, remove them from the oven. Start by scooping a layer of the ricotta mixture into each pepper half, followed by a generous spoonful of the beef and sauce mixture. Top each pepper with about 1/2 cup of shredded mozzarella cheese.
Return the stuffed peppers to the oven and bake for another 15-18 minutes, or until the cheese is bubbly and golden brown. Let cool slightly before serving.