Gently sweat 1 finely chopped shallot (1 ounce/25 grams) and 1 finely chopped medium onion (6 ounces/150 grams) in ½ ounce (15 grams) butter until they are translucent but not browned. Add 7 fluid ounces (200 milliliters) white wine and reduce by half. Add 7 fluid ounces (200 milliliters) demi-glace, 1 tablespoon (15 milliliters) tomato purée, 6 tablespoons (90 milliliters) concentrated white veal or chicken stock (four to six times), 1 ounce (25 grams) chopped unsmoked cured ham (such as prosciutto), ½ garlic clove (crushed), and 1 small bouquet garni. Slowly simmer for 30 minutes. Strain through a fine chinois. Finish the sauce with 1 ounce (25 grams) ham cut into brunoise, 1 teaspoon (5 milliliters) freshly grated horseradish, and 1 tablespoon (15 milliliters) finely chopped parsley.