In a large saute pan or skillet, heat the oil over medium-high heat. Add the onions and saute until translucent, about 4 to 5 minutes.
Then add the beef, crumbled sausage, garlic, cilantro, salt, pepper, and pepper flakes.
Cook until the meat is browned, stirring to break up the lumps.
Add the rice and marinara sauce and stir until well integrated. Then remove from the heat.
Stuff the peppers with the mixture and place in a baking dish. Bake until the peppers are tender and the filling is heated through, about 25 to 30 minutes.