1. Preheat oven to 400°.
2. Wrap shallot in foil. Bake at 400° for 35 minutes; cool for 10 minutes. Peel and mince.
3. Combine shallot, vinegar, oil, sugar, mustard, salt, and pepper in a large bowl; stir well with a whisk.
4. Add enough dressing just to coat salad mix; toss.