In a medium skillet over medium heat, cook the 1 pound ground beef, 1 small chopped red onion, and 2 minced garlic cloves until the onion is tender and the beef is no longer pink.
Stir in the 1 (15-ounce) can tomato sauce, and 1 teaspoon dried oregano. Bring the mixture to a boil. Reduce the heat and simmer for about 8-10 minutes stirring occasionally until the sauce thickens.
Place romaine lettuce on each roll. Top with the sloppy joe mixture and crumbled feta cheese.