In a skillet over medium heat, brown the steak in oil. Place steak and drippings in a slow cooker.
Add lemon juice, garlic, cumin, salt, chili powder and red pepper flakes; mix well.
Cover and cook on high for 2 1/2 to 3 hours or until meat is tender.
Add green pepper and onion; cover and cook for 1 hour or until vegetables are tender. Warm tortillas according to package directions; spoon beef and vegetables down the center of tortillas.
Top each with cheese, salsa, sour cream, lettuce and tomatoes, if desired. Fold in sides of tortillas and serve immediately.
Servings: 6 to 8