In a large skillet over medium heat, cook the Italian sausage until browned and fully cooked, breaking it into small crumbles. Drain excess grease and set aside to cool slightly.
In a large mixing bowl, combine the cooked sausage, cream cheese, 1 cup of shredded cheese, cooked rice, marinara sauce, garlic, Italian seasoning, red pepper flakes (if using), salt, and pepper. Mix until well combined.
Carefully stuff each pepper half with about 1-2 tablespoons of the filling mixture, packing it gently but firmly.
Pour about ½ cup of water or chicken broth into the bottom of your slow cooker to create moisture.
Arrange stuffed peppers in the slow cooker in a single or double layer (standing upright if possible). Drizzle with a little extra marinara sauce.
Cover and cook on LOW for 2½-3 hours, until peppers are tender but still hold their shape.
During the last 15 minutes of cooking, sprinkle the remaining ½ cup of cheese over the peppers, cover, and let it melt.
Garnish with fresh basil or parsley before serving. These are best served warm with toothpicks for easy grabbing!
