Peel and dice the onions and let them bubble up in the clear soup. Add the cornmeal. Stir and allow to bubble once, then swell on turned off stove for 10 min. Fold in Parmesan cheese (all but 2 tbsp) and butter, season with pepper. Oil a flat baking sheet, and spread the porridge on it to the thickness of a finger.
Blanch (scald), peel, seed and dice the tomatoes and crumble the feta. Spread both evenly on top of polenta. Sprinkle with marjoram and remaining parmesan. Preheat the broiler and bake the polenta for approx. 10 min.
In the meantime, rinse the romaine lettuce and cut the leaves into strips. Rinse and drain the sprouts in a sieve.
Mix sugar, vinegar, salt, pepper, garlic and oil, mix with lettuce and sprouts and spread evenly on plates. Portion the polenta and arrange on the lettuce.