Preheat the oven to 200°C.
Bake the potatoes for 1 to 1.25 hours until tender.
Pulse the onion in a food processor until finely chopped.
Gently fry the onions in 25g of butter for a few minutes.
Process the mushrooms to a fine purée and fry with the onions until the moisture evaporates.
Mix the mushroom mixture with tarragon, breadcrumbs, lamb mince, and seasoning.
Grease a sheet of foil with butter and place it on an oven tray.
Tie the lamb racks into a circle, fill with stuffing, and bake at 200°C for 50-60 minutes.
Scoop flesh from baked potatoes and pass through a fine sieve.
Mix the potato flesh with 25g of butter and egg yolks until fluffy.
Pipe the potato mixture into pyramids on a tray and bake for 5-10 minutes until golden.
Carve the rested lamb and serve with the duchess potatoes.