Sweetbread Mackenzie. Blanch and trim a good-sized sweetbread, make a little mirepoix of vegetables, paying attention to the color of the vegetables and seeing that they do not brown. Cut the sweetbread across the center, but do not halve it. Pound one ounce of pistachios and lay them in the slit of the sweetbread. Cover the sweetbread with the prepared vegetables and place in a well-buttered bag. Cook in a slow oven for thirty minutes. Dish the sweetbread, place the other ingredients in a fine sieve and lightly press the gravy through on to the sweetbread.