Brush the beet with care under cold running water. Mix plenty of salted water with the vinegar and let it boil. Cook the beet in it for about 30 minutes, then rinse, remove the skin and cut into quarters. Wash the parsley, shake it dry and pluck the leaves from the stems. Grind the parsley leaves together with the beet.
Grease a souffle dish (diameter: 20 cm for 4 portions) and sprinkle with breadcrumbs. Preheat the oven to 200 °C (gas mark 3).
Melt the butter or light butter in a saucepan. Sauté the flour in it until light. Deglaze everything with the milk while stirring and simmer gently for about 10 minutes. Stir the sour cream or crème fraiche and the beet puree into the sauce and season everything with salt, pepper and coriander.
Whip the egg whites to stiff peaks. Grate the cheese. Gently fold both into the beet mixture. Fill the souffle dish with the beet mixture. It should only be filled two-thirds full. Bake on the bottom rack for 40 minutes.