Cook beet with cumin, salt and pepper in covered water until soft, remove peel and cut into fine julienne (strips).
Cook the spaghettini in boiling salted water with a little oil until al dente and drain. Heat the butter in a frying pan, add the spaghettini and beet and season with salt and pepper.
On a heated plate, arrange the spaghettini turned up with a fork, draw a mirror with the sauce and put the fish, loosened from the bone, on top.