In a large skillet, cook beef, onion and garlic over medium heat until beef is no longer pink, 6-8 minutes; crumble beef; drain. Stir in tomato sauce and oregano. Bring to a boil. Reduce heat; simmer, uncovered, until sauce is slightly thickened, 8-10 minutes, stirring occasionally.
Place romaine on roll bottoms; top with meat mixture. Sprinkle with feta cheese; replace tops.
Freeze option: Freeze cooled meat mixture in freezer containers. To use, partially thaw in refrigerator overnight. Heat through in a saucepan, stirring occasionally; add water if necessary.