Quarter the white parts of 4 scallions lengthwise; thinly slice the green parts. Soak the scallion whites in cold water, 15 minutes; drain. Whisk 2 tablespoons rice vinegar, 1 minced garlic clove, 1 teaspoon each sesame oil, chili oil and kosher salt, and ½ teaspoon sugar. Toss with the scallions (white and green parts), 2 bunches cilantro (leaves and tender stems), 1 thinly sliced serrano chile and 1 English cucumber (cut into thin strips).