Combine 1 tablespoons oil, garlic, thyme and a pinch each salt and pepper; rub all over the pork. Cook roast on greased grill over indirect heat until internal temperature reaches 155F on meat thermometer, about 1 1/2 hours.
During the last 20 minutes of cooking, mix mustard with maple syrup and brush over roast. Meanwhile scrub potatoes; cut into wedges; toss with more oil, salt and pepper. Place in grill basket and cook over direct heat (beside roast) until tender, but crisp and golden, 30-40 minutes, shaking potatoes occasionally for even browning.
Remove roast to carving board, cover with foil and let stand for 10-15 minutes before slicing.
RHUBARB APPLE CHUTNEY:.
In a non-reactive saucepan, combine rhubarb, onion, apple, sugar, vinegar, cumin, salt, thyme, dry mustard, and pepper, Bring to a boil over medium-high heat, stirring constantly.
Reduce heat and simmer, uncovered, until rhubarb has softened but still has some texture, about 2 minutes. Let cool. Store in an airtight container, in refrigerator, for up to 2 weeks.
Makes about 1 1/2 cups.