In a bowl or jar, combine the vinegar, mustard and some salt and pepper. Whisk to combine, and then slowly pour in the olive oil while whisking. Set aside.
Trim the roots off the leeks, leaving some of the root end intact so the leeks hold together. Cut the leeks in half lengthwise and rinse very well--they are usually pretty sandy and dirty.
Add about 1 inch of water to a large pot over medium-high heat. Insert a steamer basket and set the leeks into the steamer. Cover and cook until softened and tender when the white end is pierced with a paring knife, about 5 minutes. Remove the leeks from the steamer, pat dry and transfer to a shallow dish. Brush with olive oil and season generously with salt and pepper.
Heat a grill pan or grill to medium-high heat. Add the leeks, cut-side down, and cook until grill marks form, about 3 minutes. Flip and repeat. Remove to a serving platter, drizzle over some of the vinaigrette and sprinkle with the hazelnuts.