Heat grill to medium-high. Score
the skin-side of the duck
with a knife in a lattice pattern, being sure not to cut through to the flesh. Season each breast with salt and pepper on both sides.
Rub the skin side of each breast with a few tablespoons of the rub and place on the grill, rub side down, and grill until slightly charred and the skin begins to get crispy, about 4 to 5 minutes. Turn the breasts over, brush with some of the glaze and continue grill to medium-rare doneness, another 3 to 4 minutes. Remove the duck from the grill
and brush with more of the glaze. Let rest for 5 minutes then slice 1/4-inch thick on the diagonal. Place the grilled green onions on a platter and top with the sliced duck breast. Serve with warm tortillas.