Cover the turkey slices with plastic wrap and pound to flatten. Sprinkle all over with salt and pepper, brush lightly with olive oil and reserve. Preheat the grill or broiler. Melt 3 tablespoons of the butter in a medium skillet over low heat. Cook the onions with 1 1/2 teaspoons salt, stirring and shaking the pan frequently, until the onions are golden, about 15 minutes. Pour in the vinegar, turn the heat up to high and reduce by half.
Thinly slice the remaining cold butter. Reduce the heat to low and whisk the butter into the sauce a little at a time. Remove from the heat. Stir in the cracked pepper. Grill or broil the turkey slices less than a minute per side. Serve over a bed of seared greens, if desired, with the sauce over all.
c.1997, M.S. Milliken & S. Feniger, all rights reserved