Cut meat across the grain into 1/2 inch diagonal strips. Place in a slow cook cooker. Top with onion and bell pepper. In a small bowl, combine garlic, chile, chili powder, cumin, salt, oil and lemon juice. Pour mixture over meat. Cover and cook on LOW for 6 to 6 1/2 hours. Spoon several slices of meat with sauce into center of each warm tortilla. Fold over. Top with sour cream and avocado.