Preheat the oven to 400 degrees F.
In a small bowl, mix together the mustard, chili powder, cumin, allspice, oregano, brown sugar, and garlic. Place the pork in a 9 by 13-inch roasting pan. Spread the mustard mixture over the top and sides of the pork to form a crust. Add the onions, carrots, and chicken stock to the pan. Roast for 1 hour and 15 minutes or until a meat thermometer inserted into the thickest part of the pork registers 160 degrees F. Allow the pork to rest for 10 minutes before serving. Remove the vegetables and arrange on a serving platter. Season the pan juices with salt and pepper and pour into a small serving pitcher.
To serve, slice the pork into 1/4-inch slices. Arrange the pork slices on the serving platter with the roasted vegetables and serve with the pan juices.
Cook's Note: This recipe will also work using a 2 1/2 pound beef roast. Allow an extra 10 to 15 minutes of cooking time.