Leeks marinated in a lemony vinaigrette are offered at room temperature
Make the poached leeks: Bring thyme, lemon juice, a large pinch of salt, and 5 quarts water to a boil in a large stockpot. Add leeks; reduce heat, and simmer until tender, about 15 minutes. Using tongs, transfer leeks to paper towels to drain; discard thyme and cooking liquid. Transfer leeks to a platter.
Make the vinaigrette: Whisk together mustard and lemon zest and juice in a small bowl. Add oil in a slow, steady stream, whisking until emulsified. Season with salt and pepper.
Pour vinaigrette over leeks. Let stand 30 minutes.
Leeks can be refrigerated, covered, up to 2 days; bring to room temperature before serving.