Place roast on a 18x11 inch piece of heavy-duty aluminum foil.
Stir together mustard and next 4 ingredients; spread over roast.
Fold foil over roast to seal.
Place in shallow roasting pan, and chill at least 8 hours.
Remove roast from foil; place in roasting pan. Let roast come to room temperature.
Bake covered, at 450 for 20 minutes.
Arrange uncooked potatoes (seasoned with 2 tablespoons olive oil, 2 garlic cloves pressed, 1 teaspoon salt and 1/2 teaspoon pepper sage and rosemary to taste cut into wedges) around roast, if using add pearl onions on top of potatoes and bake, uncovered, 25 more minutes or until potatoes are tender and roast is done.
Remove from oven; cover and let stand 15 minutes before slicing.
Garnish, if desired with rosemary or other herb of your choice.