Preheat oven to 350 °F.
Heat oil in a large skillet over medium-high heat and sauté ground beef for 3 to 4 minutes, until it loses most of its pinkness. Pour off fat.
Add Worcestershire sauce, garlic and chili powders, onion flakes, parsley and black pepper and cook for 1 to 2 minutes, stirring continuously.
Mix in cooked rice and 2 cups of the marinara sauce until well combined.Lightly stuff each pepper with beef and rice mixture and stand stuffed peppers upright in an ungreased 9 x 13-inch pan. Peppers should be standing upright and close together.
Pour remaining 1½ cups marinara sauce over top and around peppers.
Bake, uncovered, for 1 hour to 1 hour and 30 minutes, until peppers are soft.
Spoon any remaining sauce from the pan over peppers before serving.