Grilled Flank Steak with Coca Cola Pickled Onions

Name: Grilled Flank Steak with Coca-Cola-Pickled Onions
Ingredients: 6 tbsp. gochujang (amazon.com); 3 tbsp. mirin; 1 tbsp. plus 1/2 cup soy sauce; 2 tsp. toasted sesame oil; 4 cloves garlic, peeled; 1 (1-inch) piece ginger, peeled and thinly sliced; 1 (2-lb.) flank steak; 2 tbsp. sambal oelek (amazon.com); 1 (12-oz.) can Coca-Cola; 1⁄2 cup cider vinegar; 2 tbsp. fresh lime juice; 2 large red onions, thinly sliced; Kosher salt and freshly ground black pepper, to taste; 2 heads butter lettuce, leaves separated