4 large baking potatoes
300g/10½oz button mushrooms
1 small onion
1 small bunch picked tarragon, finely chopped
salt and freshly ground black pepper
50g/1¾oz fresh white breadcrumbs
25g/1oz butter
25g/1oz butter, plus extra for greasing
3 free-range egg yolks
75²µ/2½´Ç³ú lamb mince
2 x 7-bone French trimmed racks of lamb