To mellow the onion, place in a small bowl and add enough cold water to cover it; set aside.
Meanwhile, cook pasta in lightly salted boiling water about 8 minutes or until al dente. Drain pasta and place in a large bowl. Toss with milk. Set aside to cool slightly while preparing dressing.
For dressing, in a medium bowl, whisk together vinegar, mustard, salt, and pepper. Gradually whisk in olive oil, adding oil in a thin steady stream. Whisk until thickened. Whisk in sour cream.
Drain onion, pat dry. Add to macaroni along with ham, peas, celery, and dill weed. Add dressing; toss to coat. Serve immediately or cover and chill for 2 to 24 hours. Makes 10 servings.