Source: Rita Sodi and Jody Williams, bonappetit.com, May 2014
4 large leeks, white and pale-green parts only, tough outer layer removed
1 small shallot, finely chopped
½ garlic clove, finely grated
1 tablespoon Sherry vinegar or red wine vinegar
1 teaspoon Dijon mustard
1 teaspoon whole grain mustard
½ teaspoon finely chopped fresh thyme
¼ teaspoon sugar
¼ cup olive oil
Freshly ground black pepper
Trim root end of leeks (leave as intact as possible so they don’t break apart while cooking) and cook in a large pot of boiling salted water until meltingly tender (a paring knife should go all the way through with no resistance), 15–20 minutes. Transfer to paper towels to drain; let cool.
Whisk shallot, garlic, vinegar, Dijon and whole grain mustards, thyme, and sugar in a small bowl. Gradually whisk in oil and 1 Tbsp. water; season vinaigrette with salt and pepper.
Halve leeks lengthwise and arrange on a platter, cut side up; drizzle with vinaigrette and let sit at least 10 minutes before serving.
DO AHEAD: Leeks can be made 1 day ahead. Cover and chill. Bring to room temperature before serving.