Combine oil, mustard, thyme, salt and pepper in a large bowl. Whisk to blend
well. (Can be prepared 6 hours ahead. cover and let stand at room
temperatue.)
Prepare barbecue (medium-high heat.) Add potatoes and onions to mustard oil.
Toss to coat. Set 1 long sheet of heavy duty aluminum foil (or 2 sheets of
regular) on work surface. Spray foil with PAM. Place vegetables in center
of foil. Pour remaining oil over. Cover with more foil; roll in top and
bottoms of foil - then each end to make a package. Seal tightly. Place
package on grill; cook approximately 25 minutes. Remove from grill, slit
package top; fold back and garnish with thyme sprigs.