1. Preheat the oven to 350° and line a baking sheet with parchment paper or foil. Using a 2-inch round biscuit cutter, cut the potato slices into rounds. In a bowl, toss the potatoes with the olive oil and arrange them in a single layer on the baking sheet. Season lightly with salt and pepper, then sprinkle with the cheese. Bake the potato rounds for about 45 minutes, or until deep golden; let cool. Transfer the potato coins to paper towels.
2. In a small bowl, combine the ham with the mayonnaise, scallion, cornichons, capers, tarragon and mustard. Season with salt and pepper. Spoon the ham salad onto the potato coins and serve.