For the pumpkin jam, bring a small amount of water to a simmer in the bottom of a pan. Suspend a steaming basket over the water. (NB: Do not allow the base of the steaming basket to touch the water.)
Place the pumpkin pieces into the steaming basket and cover the pan with a lid. Steam the pumpkin for 10-15 minutes, or until tender.
Transfer the steamed pumpkin to a food processor and blend until completely smooth.
Meanwhile, grind the cinnamon, nutmeg, cloves and mace to a powder in a spice grinder or using a mortar and pestle.
Pour the puréed pumpkin into a saucepan, then stir in the sugar and ground spices. Cook over a low heat, stirring regularly, for 1¼-1½ hours, or until the mixture has thickened to the consistency of jam.
Remove the pumpkin jam from the heat and season, to taste, with salt and freshly ground black pepper. Chill in the fridge until needed.
Meanwhile, for the beef sauce, heat a non-reactive pan over a medium heat. When the pan is hot, add the butter and, when melted, add the beef trimmings. Fry the beef trimmings for 8-10 minutes, or until browned all over.
Add two-thirds of the chopped shallots and all of the garlic, the thyme, bay leaf, white peppercorns and a pinch of salt. Continue to cook for 2-3 minutes, or until the onion is golden-brown and caramelised.
Add the brandy and set alight with a match. (CAUTION: Keep clothing well clear and keep the flames well away from the eyes and face.) Allow the flames to flare up then die down. Stir well, scraping any browned bits up from the bottom of the pan to deglaze. Bring the liquid to a simmer and continue to simmer until the volume of liquid has almost completely reduced.
Add the sherry vinegar and return the mixture to a simmer, scraping any browned bits up from the bottom of the pan to deglaze again. Continue to simmer the mixture until the volume of liquid has reduced by two-thirds.
Add the red wine and return the mixture to a simmer. Continue to simmer the mixture until the volume of liquid has reduced by two-thirds.
Add the chicken and veal or beef stocks, stir well and bring the mixture to the boil. Reduce the heat until the mixture is simmering, then continue to simmer for 20-30 minutes, or until the mixture has thickened.
Strain the beef sauce mixture through a muslin cloth into a clean saucepan, then return the strained liquid to the heat and bring to a simmer. Continue to simmer until the mixture is thick enough to coat the back of a spoon, then strain again.
Season the sauce, to taste, with salt and freshly ground black pepper, then stir in the chopped shallot, parsley and capers.
Meanwhile, for the potatoes, place the beef fat, garlic bulb, thyme sprigs and bay leaves into a deep-sided frying pan over a medium heat and cook until the fat has completely melted.
Add the potatoes (they should be covered in the liquid fat), then reduce the heat until the temperature of the fat is 100C (use a cooks' thermometer to check the temperature). Continue to cook for 20-25 minutes, or until the potatoes are tender.
Remove the potatoes from the fat using a slotted spoon and set aside to drain on kitchen paper. Season, to taste, with salt and freshly ground black pepper.
Meanwhile, for the mushrooms and curly kale, heat a frying pan over a medium heat. Add half of the butter and, when the butter has melted, add the girolle and trompette de la mort mushrooms and fry for 2-3 minutes.
Cook the curly kale leaves in a pan of salted, boiling water for 1-2 minutes, or until just tender, then drain well.
Heat a frying pan until hot, add the remaining butter. When the butter is foaming, add the boiled kale and fry for a further 1-2 minutes, or until wilted.
Remove from the heat and season, to taste, with salt and freshly ground black pepper.
Meanwhile, for the onglet steaks, rub the steaks all over with a little olive oil. Season, to taste, salt and freshly ground black pepper.
Heat a griddle pan over a high heat. When the pan is hot, place the steaks onto the griddle and cook for 2-3 minutes on each side (for medium-rare), or until cooked to your liking. Remove from the pan and set aside to rest for 2-3 minutes.
To serve, carve each steak into two pieces and place them into the centre of each of four serving plates. Place a spoonful of the chilled pumpkin jam on either side of the steak. Dot some of the wilted kale on top of each steak, then spoon over the mushrooms. Place three or four potatoes alongside. Drizzle over the beef sauce.