Dry duck breasts thoroughly with paper towels. Using a very sharp knife, score fat side of duck in a diamond pattern, making slashes about 3/4 in. apart, to allow fat to render easily.
Put 1 tbsp. tea, rice, 1 tsp. kosher salt, and peppercorns in a spice grinder and pulverize into powder. Sprinkle powder on fat side of duck and pat in powder so that it sticks. Dredge underside of duck with any spilled powder.
Preheat oven to 450°. Heat a cast-iron pan (or other heavy, ovenproof pan) over high heat, 1 to 2 minutes, until it's nice and hot but not smoking. Gently place duck, skin side down, in pan. Reduce heat to low and cook 15 to 20 minutes, or until most of fat has rendered and has formed a crisp, deeply browned crust (it will look brown rather quickly due to the spices, but keep going until it's crisp).
Season meat side of duck with a little kosher salt and, using tongs, gently turn over. Slide pan into oven and roast 4 to 5 minutes for medium-rare, or a few minutes more for medium or well done (cut to check). Avoid overcooking; meat will be tough.
Remove duck from oven and transfer to a cutting board to let rest 5 minutes. Meanwhile, pulverize remaining 1 tsp. tea in a spice grinder, then add sel gris and give it a few pulses to combine.
Slice duck thinly, transfer to warmed plates, and sprinkle with tea salt to taste.