Wash the sole fillets and pat them dry with kitchen paper. Clean the onions and cut them into rings. Place 100 ml of water in a pan and add the onions, a good dash of olive oil, a dash of white wine, pepper and paprika powder and 1 large tablespoon of Marne Groninger Mustard. Bring everything to the boil and then lower it with a lid on the pan so that the onions can soften. Place the tongues in the sauce for 10 minutes until they are deliciously cooked. Roll up the tongues and place them (2 per person) on a preheated plate. Finish it all off with the sauce and the onion rings.