Peel and dice the onion.
Drain the capers, anchovies and gherkins and finely chop them separately.
Place the beef tartare in a bowl. Add the onions, finely chopped capers, anchovies and gherkins, mustard and ketchup.
Mix thoroughly. Add the egg and 1 tbsp parsley and season with salt and pepper.
Place 2 slices of roast beef, slightly overlapping each other, on a board. Place a spoon of tartare on top and roll tightly into a small roll. Cover and chill in the fridge.
Mix the crème fraîche, mayonnaise, remaining parsley and lemon zest in a bowl. Sprinkle with the cayenne pepper. Stir in the lemon juice and season to taste.
Serve the roast beef rolls on hash browns and garnish with the sauce and chives.
Place the rest of the tartar on 4 plates using cooking rings to make a neat shape. Chill in the fridge.
Remove the bread crust, cut into cubes, and fry in hot butter until golden brown all over. Cool, then place on top of the tartare rings.
Wash the vegetables and chop decoratively (eg. into diamonds).
Mix the vegetables in a bowl with lemon juice, oil, salt and pepper and serve around the tartare rings.