Chicken Thighs Marinated with Homemade Ssam Paste

Name: Chicken Thighs Marinated with Homemade Ssam Paste
Ingredients: 1/3 cup yellow miso; 1/4 cup plus 2 tablespoons sugar; 1/4 cup plus 2 tablespoons toasted sesame oil; 1/4 cup Asian fish sauce; 1/4 cup white vinegar; 1 1/2 tablespoons Korean red chile powder (gochugaru); see Note; 4 garlic cloves, minced; 2 tablespoons minced fresh ginger; 8 large boneless chicken thighs, with skin; 1 medium onion, sliced 1/2 inch thick; Vegetable oil, for brushing; 4 scallions, thinly sliced; Bibb or Boston lettuce leaves, for serving