To mellow the minced onion, soak it in cold water while you make the salad.
Bring a large pot of cold water to a boil over high heat and salt it generously. Add the macaroni and boil, stirring occasionally, until al dente, about 8 minutes. Drain the macaroni in a colander, put in a serving bowl, and toss with the milk. Allow the macaroni to cool slightly while you make the dressing.
For the dressing: Whisk the vinegar, mustard, the 2 teaspoons salt, and black pepper to taste in a large bowl. Gradually whisk in the olive oil, starting with a few drops and then adding the rest in a steady stream to make a smooth, slightly thick dressing. Whisk in the sour cream.
Drain the onions, pat dry, and add to the macaroni along with the ham, peas, celery, and dill. Add the dressing and fold to coat the pasta evenly. Serve immediately or cover and refrigerate until ready to serve.
Know-How: Why the milk? When we left it out of the recipe, the macaroni absorbed too much of the dressing and the salad was overly acidic. Adding just a couple tablespoons mellowed the dressing and gave the salad a wonderful old-fashioned quality without being goopy or heavy.
From Food Network Kitchens Get Grilling, Meredith, 2005