To make the mushroom-crusted lamb, dust the lamb loin lightly with flour. Dip in the beaten egg, then roll in the mushroom powder to coat. Wrap tightly in cling film and chill in the fridge for 30 minutes.
Preheat the oven to 220C/200C Fan/Gas 7. Heat a little olive oil in a frying pan. Unwrap the lamb and fry until browned all over. Transfer to a baking tray and bake for 5–10 minutes, or until cooked to your liking. Remove from the oven and set aside in a warm place to rest.
To make the cauliflower couscous, put the cauliflower in a food processor and pulse until it is very finely chopped. Tip it into a fine sieve and steam over a saucepan of boiling water for 5 minutes, or until tender. Transfer the cauliflower to a large bowl and mix in the raisins, olives and pine nuts. Season to taste with salt and pepper.
Preheat the grill to medium–high. To make the creamy mushrooms, dot the Portobello mushrooms with some of the butter and grill until tender. Set aside to cool slightly, then slice.
In a frying pan, add the remaining butter and heat until it turns a nut brown colour. Add the wild mushrooms and rosemary and cook until the mushrooms are tender.
Add the Portobello mushrooms and cream and season with salt and pepper. Simmer until the cream is reduced and thickened, then stir in the vinegar and soy sauce. Simmer further until the cream just coats the mushrooms. Sprinkle in some of the chopped parsley.
To serve, carve the lamb into four slices. Spoon some of the mushrooms onto a serving plate and place a slice of lamb on top. Serve with the cauliflower couscous.