Trim excess fat from beef and cut into 2" pieces. Combine with onion and garlic in a 4-5 quart slow cooker. Mix together fajita seasoning mix and undrained tomatoes and pour over beef. Place peppers on top. Cover crockpot and stir well. Cover slow cooker and cook on high for 15-20 minutes until thickened, stirring occasionally. Serve stew with hot cooked rice or hot couscous.