First, line a standard sized buttered terrine mould with clingfilm. Heat the duck fat in a heavy-bottomed pan, keeping at a temperature of 100-130°C. Place the pheasant breast into the fat and cook for 1 hour. Slice each breast into 3 and keep warm.
Soak the leaf gelatine in cold water. Heat the butter and add the soaked gelatine, whisking until the mixture emulsifies. Dip the pheasant breast pieces into the butter/gelatine mixture and place a layer into the terrine. Repeat the process with the savoy cabbage, beetroot, then with the pheasant breast, cabbage and beetroot again, and so on. Place another layer of clingfilm over the terrine and press overnight.
Mix the white wine vinegar with the sugar, until the sugar dissolves, add the mustard and whisk in the oil. Add the walnuts and chopped quince.
When ready to serve, slice the terrine and place a slice in the centre of the plate, drizzle the dressing around and garnish with fresh garden herbs.