Heat the olive oil in a small Dutch oven over medium heat. Add the onions and mushrooms then cook for 3-4 minutes. Add the ground beef, bell pepper, chopped garlic then season with sea salt and freshly cracked pepper; cook for 4-5 minutes making sure to crumble the ground beef as it cooks. Drain any excess grease from the pan then add the marinara sauce, stir until mixed thoroughly. Simmer the sauce for 1-2 hours so flavors combine.