To make the reduction, cook all the chopped vegetables in butter with a lid on for 10 minutes with the garlic and thyme. Add the white wine and boil until reduced by two-thirds. Add the chicken stock and continue to boil until reduced by half. Keep to one side.
To start the sauce, melt the butter in a pan, stir in the flour and cook over a low heat for 10 minutes. Warm the milk in a sep