Peel the onion. Cut onion, cucumber and bacon into fine cubes. Heat oil in a frying pan. Fry the bacon for 2-3 minutes until crispy. Fry the onion briefly. Add the gherkins and let everything cool down a little.
Wash the fish, dab dry and cut into 4 pieces. Season with salt and pepper. Spread mustard and tomato paste on one side. Put 1 tablespoon of bacon mixture on one half of each fish fillet.
Fold up the fillets, pin them down and place them in a greased casserole dish.
Stir the crème fraîche and milk until smooth. Season with salt and pepper and pour over the fish. Grate Gouda and sprinkle with the remaining bacon mixture. In a preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see
manufacturer) cook for about 45 minutes.
Cook rice in approx. 400 ml boiling salted water according to package instructions. Wash and finely chop the dill. Sprinkle fish with dill and serve with rice.