While the leeks are cooling, add 1/3 cup olive oil, 1 tbsp red wine vinegar, 1 tspn Dijon mustard, 1 tspn grainy mustard, 1 clove garlic, finely grated and 1 whole shallot finely chopped to a jar with a tight fitting lid. Shake well to combine.
Add a generous tablespoon of the vinaigrette to a casserole dish and spread the vinaigrette to cover the bottom.
Remove the outside layer of each leak, then place the leek halves on the vinaigrette, cut side down. Alternate root end to cut end.
Drizzle the remainder of the vinaigrette over the leeks and refrigerate for 3 hours, or overnight.