Wash the chicken thighs in cold water and pat dry with paper towels. Make shallow cuts and use the spine of the knife to gently pound the chicken to an even thickness. Season evenly with salt and black pepper.
2
Prepare the sauce: Heat 2 tablespoons of oil and add 1 teaspoon of red chili powder to infuse the oil. Strain and pour into a bowl. Mix in soy sauce, vinegar, sugar, minced garlic, mirin, and sesame oil, stirring until the sugar is completely dissolved.
3
Prepare the batter: Mix potato starch, pinches of salt, and a large egg in a bowl until combined.
4
Prepare the vegetables: Slice the Serrano peppers into very thin strips. Cut onion into fine matchsticks. Cut green onions into 2-inch pieces and soak in cold water to reduce spiciness.
5
Heat 2 cups of oil to 350°F over medium-high heat. Lightly coat each piece of chicken in the batter and remove excess.
6
Fry the chicken for about 5 minutes. Remove and drain. Heat the oil again for 1 minute.
7
Return the chicken to the oil for a second fry for about 5-6 minutes until very crispy. Transfer to a strainer and let cool slightly.
8
Cut the cooled chicken into bite-sized pieces. Arrange lettuce on a plate as a base. Top with chicken, green onions, onion strips, and sliced peppers. Pour the sweet and sour sauce over everything and toss gently before serving.