Preparation

• Heat the oil in a 2-quart saucepan over medium heat. Add the shallot, carrot, celery, and 1/2 tsp. salt and cook, stirring occasionally, until the vegetables are beginning to brown around the edges, about 6 minutes. Turn the heat down to medium low, add the tomato paste, and cook, stirring, until the tomato paste has thickened and is a few shades darker, another 2 to 3 minutes. Add the vinegar, turn the heat up to high, and boil gently until thick again, about 4 minutes. Stir in the Worcestershire sauce. Add the stock and the rosemary, bring to a boil, and boil gently until the sauce is reduced to about 2 cups, about 25 minutes.
• 在一半的玉米淀粉浆搅拌,煮,whisking, for a minute. For a thicker sauce, whisk in a bit more of the slurry; if the sauce is too thick, add a little more stock. (If making the sauce ahead, take it off the heat, let cool, and refrigerate for up to 1 day; reheat gently.) Whisk in the capers, parsley, and butter until incorporated and season to taste with salt and pepper.

Make Ahead Tips

The sauce may be made 1 day ahead up to the point indicated in the recipe, but be sure to allow the sauce to cool before covering and refrigerating. Reheat gently and finish as directed.