Preheat the oven to 425°F. To cook the beef, heat a small ovenproof frying pan until very hot. Season the beef and rub in the mustard powder. Add the olive oil to the pan and then brown the beef for 1 minute on each side. Transfer to the middle shelf of the oven for 10 minutes until medium rare or cook a little longer if that is your preference.
To make the Yorkshire puddings, generously brush 2 x 12-hole a mini muffin tray with the sunflower oil and place on the top shelf of the oven. Place the flour in a bowl with a pinch of salt. Make a well in the centre and break in the eggs. Add half the milk and beat until smooth. Then add the remaining milk, mixing until well combined. Remove the tray from the oven, season well and divide the batter between the moulds. Cook for 10-12 minutes until well risen.
Mix together the mustard, the horseradish and sugar. Grate in the zest of half of the lemon and squeeze in a teaspoon of the juice. Whip the cream to soft peaks and then fold in the horseradish mixture. Set aside.
Remove the beef from the oven and leave in a warm place to rest. Crave the beef into thin slices and arrange on top of the Yorkshire puddings. Add a dollop of horseradish cream to each one and garish with a sprinkling of fleshly ground black pepper and a sprig of watercress to serve.