Heavily season the entire roast with kosher salt and pepper. Wrap the roast tightly with plastic wrap and refrigerate for 24 hours. Remove the roast from the refrigerator, rinse off the salt and pat dry. Set aside. In a blender, combine the onions, garlic, rosemary, mustard and oil. Blend until smooth. Season with salt and pepper. Using a pairing knife, make several slits in the veal roast. Stuff the slits and rub the entire veal roast with the mustard mixture.
Place on a rack in a roasting pan and cover tightly with plastic wrap. Refrigerate for 24 hours. Remove the roast from the refrigerator and discard the plastic wrap. Bring the roast to room temperature.
Preheat the oven to 425ºF.
Place in the oven and roast for 30 minutes. Reduce the heat to 350ºF and continue cooking for about 3 hours for medium rare. During the last half-hour of cooking, check the roast occasionally with a meat thermometer, when the temperature reaches 135ºF, remove from the oven. Allow the roast to rest for about 20 to 25 minutes for the perfect medium-rare.
The cooking times may vary because one oven may cook differently then the other. Also if using a convection oven, the cooking time will be less. So that is why it is important to invest in a good meat thermometer instead of relying on cooking times.
Carve the roast into individual servings and serve with the potatoes.
Yield: about 10 to 12 servings