Preheat oven to 400°F. Toss the rhubarb and red onion with ginger, garlic, and oil. Distribute on a sheet pan with parchment paper. Roast for 10 minutes.
Place a cast-iron skillet on the stovetop over medium-high heat. Add oil and butter. When the butter foams, add the chicken skin-side down and sear until golden brown, about 5 minutes. Turn the chicken over and sear for another 2 minutes.
While the chicken is on the stove, combine rhubarb and 2 tablespoons honey in a medium bowl. Add soy sauce and toss to combine. Add a pinch of salt and pepper, and toss with rhubarb to coat evenly.
Remove chicken from oven and carefully scoop rhubarb over the chicken. Combine remaining until the chicken is cooked through, and the rhubarb is very tender, about 15 to 20 minutes, turning the rhubarb and chicken halfway through.
Remove the pan from the oven and let rest for 5 minutes before serving. Taste and adjust seasoning with salt and pepper. Serve hot with steamed rice. Drizzle with a pinch of fresh herbs, if desired. Serve with rhubarb-onion mixture on the side.
