Preheat the oven to 150C/130C fan/gas 2.
Heat half of the vegetable oil in a medium saucepan over medium heat. When hot, add the shallots, garlic and a pinch each of sea salt and pepper and cook for about 7 minutes until soft. Add the Madeira and simmer for 5-7 minutes until it has a syrupy consistency, then add the stock, thyme and dried porcini mushrooms and summer gently for 30 minutes.
Remove the thyme stalks and transfer the sauce to a blender or food processor. Blitz until smooth, pass through a fine sieve into a clean saucepan and add the cream. Keep warm until ready to serve.
While the sauce is simmering, heat the butter over high heat in a frying pan large enough to fit the monkfish tail(s). Once it’s foaming, season the monkfish liberally with salt, then place into the hot butter. Brown the tail(s) all over for about 5 minutes in total, then transfer to a roasting dish. Pour the butter over the top and place in the oven for 25 minutes.
Turn the tail(s) over and cook for a further 25 minutes. Remove from the oven, cover with foil and leave to rest for 10 minutes. Turn the heat on the oven up to 220C/220C fan/gas 7. Squeeze the lemon juice over the monkfish and place back in the oven for 8 minutes.
Meanwhile, heat the remaining tablespoon of vegetable oil in a separate large frying pan. When hot, add the mushrooms, season well and cook over high heat for about 10 minutes (you might have to do this in batches, depending on the size of your pan).
Carve the monkfish either by using a serrated knife, and cutting across the bone, or by slicing down either side of the bone and portion accordingly. Serve the fillets atop the mushrooms and smother with the sauce.