Place celery, garlic, tomato paste, bouillon cubes, and water into a large Dutch oven; stir until the tomato paste has dissolved. Add peppercorns and bay leaves, place over medium heat and bring to a simmer.
Meanwhile, heat oil in a large skillet over medium-high heat. Add oxtail and cook until browned on all sides, about 10 minutes. Remove oxtail from hot oil and place into Dutch oven. Pour out all but 1 tablespoon of oil from the skillet, reduce heat to medium, and cook the onion until softened and translucent, about 5 minutes; add to oxtail.
Reduce heat to medium-low, cover, and simmer for 2 1/2 hours. Season with salt and pepper, recover, and continue to cook until the oxtail is tender, but not falling off of the bone, about 30 minutes.
Remove oxtail pieces and place into a serving dish. Add kidney beans to Dutch oven and return to a simmer. Thicken with cornstarch dissolved in water, simmer for 1 minute until thickened and clear. Pour sauce over the oxtail.
