Season the beef with salt and pepper and sear on both sides for 10 seconds
Leave to cool then dice into 1cm pieces
Mix the beef with the shallots, gherkin, capers, beef fat and mix well, then season with salt and pepper
Place a 10 cm ring on a plate and fill the ring with a quarter of the beef mix. Repeat with the remaining beef tartare
Squeeze some mustard on top of each ring of beef, then cover with the potato crisps
Grate the Gruyère cheese over the tartare, then place under the grill for 30 seconds to melt the cheese, and serve