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 1/4 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 1/4 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.