Add the chicken stock together with the vinegar, mustard powder, cayenne pepper, tarragon, orange zest, orange juice and mushrooms. Stir everything well and season to taste with sea salt and freshly ground black pepper.
1 cup chicken stock, 1 tbsp white wine vinegar, A pinch of mustard powder, A pinch of cayenne pepper, 2 tsp fresh tarragon, Zest of 1 orange, 1 tbsp orange juice, ¼ lb mushrooms, Sea salt and freshly ground black pepper to taste