Combine in a large bowl or jar the shallot, Dijon mustard, salt, lemon juice, and red-wine vinegar. Whisk together to combine well. Slowly whisk in 1/2 cup extra-virgin olive oil. Season with salt and pepper to taste.
Store jar in refrigerator if making ahead and shake well before using.
Serve with 2 large heads Bibb, butter, or romaine lettuce that has been washed and dried well. Tear lettuce leaves into bite-size pieces and toss with dressing.