Recipe courtesy of Bon Appetit. Click here for full instructions.
Preheat oven to 500°. Line a baking sheet with foil. Pulse chile, garlic, and white parts of scallions in a food processor until finely chopped. With machine running, drizzle in soy sauce, then olive oil; process until emulsion forms. Transfer sauce to a bowl.
Place chicken thighs, skin side up, on a work surface and slash each crosswise at 3/4-inch intervals down to the bone. Season lightly with salt. Place on prepared baking sheet and brush with sauce. Bake until skin is crisp and an instant-read thermometer inserted into thickest part of thigh registers 165°, 20–25 minutes. Broil on high for an additonal 2–3 minutes for crisper skin, if desired. Let rest for 5–10 minutes.
Meanwhile, toss rhubarb, next 5 ingredients, and green parts of scallions in a medium bowl to coat. Season to taste with salt and pepper and let stand for at least 10 minutes to allow flavors to meld.
Serve chicken with rhubarb salsa alongside.