Place the chilies, lemongrass paste, fish sauce, sugar, salt, cumin, scallions, kaffir lime leaves, garlic, and ginger into a food processor. Pulse until well chopped.
Place the buns on plates. Top with the salad leaves and strips of carrot. Add the burgers, cucumber strips and onion slices, then drizzle on the chili sauce before putting the top of the bun on the burger.