Preheat the oven to 180°C/gas mark 4
To begin, make the potato purée. Wash the potatoes, sprinkle with salt and bake in a preheated oven until soft in the middle, for about 2 hours.
Remove from the oven and whilst still hot, cut them in half and scoop out the soft flesh. Pass the flesh through a sieve quickly with a spatula. In a heavy-based pan, bring the cream and butter to the boil. Add the sieved potato and beat over the heat with a wooden spoon. Season the purée with salt and pepper, then add the olive oil and beat again until all of the oil is incorporated. Cover and leave in a warm place until needed.
To make the red wine sauce, place a pan over a medium heat. Once hot, add the beef trimmings and brown well to colour and release the fat. Add the shallots, garlic and thyme and cook until everything is nicely caramelised.
Add a knob of butter, allow to coat and caramelised a little more, then add the red wine to deglaze and allow to bubble away for a few minutes. Add the veal stock, allow to come to a simmer then reduce the heat and allow the sauce to bubble gently for 20 minutes.
To prepare the onions, add a dash of oil to a hot frying pan and add the peeled onions. Cook, shaking the pan occasionally, until the onions are nicely coloured. Add a large knob of butter, followed by the garlic and thyme. Add the chicken stock and cook down until reduced, shaking the pan to coat the onions in the stock. By the time the stock has cooked down, the onions should be cooked through. Pass the sauce through a piece of muslin into a clean saucepan and season to taste.
Add the pancetta to a hot pan with a tiny drop of oil and colour nicely, tossing until golden brown all over. Add the girolles, cook until golden, then add the trompettes and cook down. Sprinkle with the parsley, salt and pepper, then add the silverskin onions to complete the garnish.
Season the onglet and place on a hot grill, occasionally brushing with clarified butter. Grill both sides until medium rare – this should take about 3–4 minutes – then remove from the grill and rest for 2 minutes in a warm place.
To serve, place a spoonful of potato purée on a plate, slice the onglet and place over the top of the purée. Place a generous spoonful of the mushroom and mushroom mixture over the top and finish with a little red wine sauce.