Place the leeks in a bowl, sprinkle generously with salt, add the vinegar and 2 tablespoons water and toss. Cover and let marinate for 30 minutes to 1 hour.
Peel the oranges, halve them, discard any pips and remove the hard central membrane. With a sharp knife, carefully cut each half into 1/2-inch slices. Arrange on a shallow platter and add the olives.
Drain the leeks and press between double layers of paper towels to dry. Sprinkle the leeks over the sliced oranges.
Make the dressing: In a bowl, whisk together the olive oil, verjus, a pinch of salt (keep in mind that olives are usually quite salty), the Mara pepper and the tarragon. Taste, correct the seasoning, pour over the salad and serve.
You can substitute 2 tablespoons fresh lemon juice and 1 tablespoon white wine for the verjus, if you like. If you cannot find fresh tarragon you can substitute 3 tablespoons mint leaves.
"Mediterranean Vegetarian Feasts" by Aglaia Kremezi (c) Stewart, Tabori & Chang 2014. Reprinted with permission from Stewart, Tabori & Chang. All rights reserved.