In small bowl, mash orange zest and thyme into paste with back of spoon. Stir in mustard and brown sugar. Let stand at room temperature 1 hour.
Preheat grill. Lightly brush one side of lamb chops with about quarter of the mustard mixture, dividing it evenly.
When fire is hot, lay chops on rack, mustard side down. Cover and grill 2 minutes.
Brush chops with quarter of mustard, turn, cover and grill another 2 minutes.
Brush and turn chops twice more, grill until mustard mixture is used up, about 10 minutes for rare, or until done to your liking.
Season with salt and pepper. Makes 4 servings.