In a pan with half of the butter and a drizzle of olive oil, lightly brown the finely chopped onion and celery. Add the rice, toast well, and then mix in the eel previously cleaned, skinned and cut into small chunks and boiled for about 30 minutes. Continue cooking the rice, adding when necessary the cooking liquid from the eel. Stir in the tomato paste about half-way through cooking and finally, off the heat, cream with butter and cheese and serve immediately.