In a small bowl, mix together the mustard, garlic, cumin, oregano, allspice, chili powder, brown sugar and oil. Place the turkey breast in a 9 by13-inch roasting pan. Spread the mustard mixture over the top and sides of the turkey to form a crust. Add the onions, carrots, and chicken stock to the pan. Roast for 45 minutes. Cover the pan loosely with foil and continue to bake for another 45 minutes or until an instant-read thermometer inserted into the thickest part of the breast registers 175 degrees F. Allow the turkey to rest for 20 minutes before serving.
Remove the vegetables and arrange on a serving platter. Remove the turkey and place on a cutting board. Pour about 1/2 cup of the pan juices into a small saucepan. Whisk in the flour until smooth. Whisk in the remaining pan juices. Bring the mixture to a simmer over medium-high heat. Cook, stirring occasionally, for 5 minutes until the mixture thickens. Season the pan juices with salt and pepper and pour into a serving pitcher.
To serve, slice the turkey into 1/4-inch slices. Arrange the turkey slices on the serving platter with the roasted vegetables and serve with the pan juices.