Cut the onion into cubes and the beetroot into strips. Fry the onion in a pan in a little olive oil until translucent, then deglaze with the juice of the beetroot. Add the beetroot strips, horseradish and cream and simmer briefly. Salt the whole thing a little.
Put the defrosted salmon, lightly acidified with lemon juice, in a casserole dish that has been greased (e.g. with olive oil). Pour the beetroot with sauce over it and sprinkle with some grated cheese. Cook for approx. 20 minutes in the oven preheated to 180 ° C (top / bottom heat, convection: approx. 160 ° C).
Of course, fresh beetroot also works, which then has to simmer a little longer. If you only find cream horseradish, you save the cream and use more of it. The whole thing goes wonderfully with pasta of all kinds or rice.