For gratinated plaice fillets, first peel and finely chop the onion. Drain the beans well. Salt and pepper the plaice fillets on both sides. Cut the mozzarella balls into not too thin slices.
Melt a little butter in a pan and sauté the onion cubes in it. Add the beans. Add salt, pepper and a little sugar. Deglaze with balsamic vinegar and add a dash of white wine. Let the sauce boil down a bit.
In the meantime, line a baking tray with baking paper and place the plaice fillets on it. Spread the mozzarella on the fish fillets and bake at 170 °C for 10-15 minutes.
Serve the baked plaice fillets with the beans.