Salmon Burgers with Pickled Cucumbers

Name: Salmon Burgers with Pickled Cucumbers
Ingredients: 1½ pounds boneless, skinless center-cut salmon, patted dry; 5 scallions, green parts finely chopped, white parts thinly sliced; 1 1-inch piece ginger, peeled, finely grated; 1 garlic clove, finely grated; 2 tablespoons plus ⅔ cup mayonnaise; Kosher salt; 1 teaspoon toasted sesame oil; 4 teaspoons unseasoned rice vinegar, divided; 3 medium Persian cucumbers, shaved lengthwise; ½ serrano chile, very thinly sliced crosswise; 1 teaspoon sugar; ¼ cup (or more) vegetable oil; ½ cup rice flour; 2 cups tender herbs, such as torn mint and/or cilantro leaves with tender stems; ¾ cup trimmed watercress; 2 teaspoons toasted white sesame seeds (optional); 4 brioche buns, lightly toasted