In a skillet over medium heat, brown the steak in oil. Place steak and drippings in a 3-qt. slow cooker. Stir in the lemon juice, garlic, cumin, seasoning blend, chili powder and red pepper flakes. 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.