Step 1

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.

Step 2

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.

Step 3

Halve leeks lengthwise and arrange on a platter, cut side up; drizzle with vinaigrette and let sit at least 10 minutes before serving.

Step 4

DO AHEAD: Leeks can be made 1 day ahead. Cover and chill. Bring to room temperature before serving.