Cut the matjes fillets into large cubes. Peel the onion, cut in half and slice the halves. Cut the cucumber into large cubes. Mix everything in a bowl with the mustard seeds.

For the marinade, put all the ingredients (except for pepper and salt) in a tall, narrow container and beat with a hand blender. It gets a consistency that resembles a mayonnaise that is not too firm and should taste slightly sweet-sour-spicy. Only now season with salt and pepper. Be careful with salt, it depends on how salty the herring is. Then mix the marinade with the fish. Seal the bowl and let it steep in the refrigerator for at least 4 hours (overnight is better).

Go with jacket potatoes and a boiled or fried egg. ................. 'n good'n.