Step 1Lay the veal slices on wax paper. Coat each side with the mustard and sprinkle with thyme. Place a second sheet of wax paper on top and pound the veal until uniformly thin.
Step 2Arrange a bed of mesclun on each of 4 serving plates and set aside.
Step 3Heat the oil in 2 large skillets over medium-high heat until very hot but not smoking. Season the veal well with salt and pepper. Add to the skillets and cook until seared on one side, about 30 seconds. Turn the veal and cook the second side until browned around the edges but still slightly pink inside, 1 to 1 1/2 minutes more; do not overcook. Divide the veal among the serving plates. Immediately add the vinegar to the hot skillets and swirl, then pour over the veal and mesclun and serve.