Cut the fillet in 2 steaks and grill on a pre heated griddle pan, remove the bones off the lamb rack and seal whole in a sauté pan than finish in the oven until medium cooked. Using the same pan add the salted butter and chopped garlic brown than add the wine and stock, reduce until sauce in thick enough. Melt the butter sauté the chopped spring onions without browning, add the peas, mint and mash, add a little water if mixture is too dry. Slice the oyster mushrooms and saute with the rosemary. Cut the potato in 2 large cubes and using a Parisian spoon make a whole on top of both, than boil until cooked. Dice the pepper, courgette and onion very small and sauté together than fill the whole made with Parisian spoon once potato in cooked. Serve the pea puree underneath a thick slice of lamb and the oyster mushrooms on top of the fillet with the stuffed potato in the middle drizzled with the reduced sauce.