Put 2 tablespoons chopped toasted walnuts in a bowl and crush with a wooden spoon. Mix with 1 tablespoon walnut or olive oil and 1 1/2 teaspoons balsamic vinegar, then 4 cups rinsed and crisped baby salad greens (4 oz.) such as mâche, frisée, and/or arugula, and salt and pepper to taste. Mound greens mixture on one end of each of 15 slices prosciutto (6 oz. total) and roll up tightly.