Combine oil, mustard, thyme, salt and pepper in large bowl. Whisk to blend well. (Can be prepared 6 hours ahead. Cover and let stand at room temperature.)
Prepare barbecue (medium-high heat). Add potatoes and onions to mustard oil. Toss to coat. Set six 18 x 9-inch sheets of heavy-duty aluminum foil on work surface. Spray foil with nonstick vegetable oil spray. Divide vegetables among foil sheets, placing in center of left half of each. Sprinkle with salt and pepper. Fold right half of foil over vegetables. Fold edges of packages together to seal tightly.
Place packages on grill. Grill until potatoes are tender and golden brown, turning occasionally, about 25 minutes. Remove packages from grill. Slit top of foil and fold back. Garnish potatoes with thyme sprigs, if desired.
