For the onion purée, peel and slice the onions, then add to a pan with the butter and thyme. Slowly caramelise for approximately 4 hours on a very low heat. When the onions are caramelised stir in the double cream and bring to boil. Remove the pan from the heat, take out the thyme and place the rest of the mixture into a blender. Add the milk and salt to the blender, blitzing until smooth. Pass the purée through a fine sieve into a container and place in the fridge until you are almost ready to serve.
Season the Roscoff onions with salt and olive oil, then wrap in foil and bake at 190ºC/gas mark 5 for 15 minutes. Take out of the oven and set aside to cool. When the baked onions are cool peel the skin and slice off the top. Remove the soft flesh from the inside and keep the two outer layers to be filled later.
To begin making the filling, place the shallots in a pan with the butter and sweat until tender. Add the girolles to the pan and flash fry on a high heat before deglazing with the sherry. Stir in the cream, simmering to reduce the liquid by half. Add the parsley, chives and tarragon to the sauce, then spoon the mixture into the empty onion layers. Sprinkle the Parmesan and the breadcrumbs over the top and grill on a high heat until golden brown.
Preheat the oven to 180ºC/gas mark 4. Remove the beef rump from the fridge to bring it up to room temperature. Season the beef rump with a lot of salt and pepper. Sear all sides on a very high heat. Transfer the rump into the oven and cook until it reaches a core temperature of 37ºC , which should take about 15 minutes.
While the beef is cooking, preheat a deep fat fryer to 150ºC to prepare the onion rings for the garnish. Finely slice the onion into 1.5 mm rings, using either a sharp knife or mandoline. Mix the egg white with the salt and dip the onion rings into the egg mix before coating in the flour. Deep fry until crisp, then set aside on kitchen paper until ready to serve.
Take the beef out of the oven and leave to rest in a warm place for another 15 minutes. While the beef is resting , warm up the onion purée on a low to medium heat.
When ready to serve, place a stuffed onion on to each plate followed by a spoonful of the warm onion purée. Slice 4 large portions of the beef rump against the grain and add to each plate. Garnish with land cress and the crispy onion rings.
Recipe courtesy of www.greatbritishchefs.com