Heat oven to 20)C/180C fan/gas 6. Rub the beef with the oil and mustard, then season. Heat a non-stick frying pan over a medium-high heat. Sear the beef for 2 mins each side and place in the oven for 10 mins for rare and 12-15 mins for medium rare (you may have to adjust timings if your piece of beef is especially thin or thick). Remove from the oven, cover with foil and leave to cool.
Mix together the crème fraîche, chives and horseradish in a small bowl and season to taste. Keep in the fridge until ready to serve.
Just before serving, arrange the beef on a chopping board with the Stilton, rocket and crème fraîche. Toast the bread and serve while still warm. Toast the bread and serve while still warm. Top the crostini with rocket, slices of beef, chunks of cheese and dollops of crème fraîche.