Slow Cooker Korean-Style Beef Tacos

Name: Slow Cooker Korean-Style Beef Tacos
Cuisine: Korean, Asian
Ingredients: 1.25 pounds flank steak; 1/4 cup soy sauce; 1 tsp sesame oil; 1/4 cup onion; 3 garlic cloves; 1 tbsp brown sugar; 1/2 tbsp fresh ginger; 1/2 tsp crushed red pepper flakers; 2 cups cucumbers; 1 cup shredded carrots; 1.5 tbsp rice wine vinegar; 1 tsp sesame oil; 6 corn tortillas; 2 tbsp gochujang; 1 cup romaine; 1 scallion