Thinly slice cucumbers and place in a colander. Sprinkle with 1 teaspoon kosher salt, toss, and let sit for 10 minutes. Pat dry to remove excess moisture.
Warm 2 tablespoons neutral oil in a small pan, add red pepper flakes, stir for 20–30 seconds, then remove from heat and let cool slightly.
Combine rice vinegar, soy sauce, sugar, sesame oil, garlic, lime juice (if using), and the prepared chili oil. Whisk until sugar dissolves.
Combine drained cucumbers, shallot, and scallions in a bowl. Pour dressing over and toss to coat evenly.
Garnish with toasted sesame seeds and chopped herbs. Serve immediately or chill for 15–30 minutes for deeper flavor.
Notes
To keep cucumbers crisp for meal prep, store dressing separately and toss just before serving.
Swap half the soy sauce for fish sauce for more umami, or add yogurt for a creamy version.
Adjust heat with more or less chili oil depending on preference.