Finely crush juniper, pepper and mustard seeds in a mortar. Add oil, lemon juice and 1 pinch of salt. Peel onion, dice finely and mix with the spice paste. Wash the fish fillet, dab dry and cut into 8 pieces.
Place in a greased, wide ovenproof dish and score across several times. Spread the spice paste over it. Cook in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 20 minutes. Clean and wash the spinach.
Peel and chop the garlic. Wash the tomatoes and cut into slices. Steam everything in hot fat for approx. 5 minutes (cover frozen spinach with 100 ml water for approx. 15 minutes). Season with salt and pepper.
Arrange everything, possibly garnish with lemon and parsley. Rice tastes good with it.