Preheat the oven to 190ºC. Lay the duck breasts skin side down on a cutting board. Use a sharp knife to trim off the excess skin so that the remaining skin is the same shape as the breast. Reserve the trimmings for cooking.
Lightly score the skins of the duck breasts using a sharp paring knife; score the skin diagonally 5 mm apart. Rotate the breasts 90° and score again, intersecting the previous lines to make a crosshatched pattern.
Sprinkle salt and five-spice onto a sheet tray or baking pan.
Lay the duck breasts skin side up in the pan. Season heavily with more five-spice and salt and pepper to taste. Rub the duck breasts evenly in the salt and five-spice seasoning and let rest for 2 minutes before searing.
Heat a large cast-iron pan on low for 3 minutes. Lay the duck breasts in, skin side down, and gradually increase the heat to medium. Add the skin trimmings to the pan. Sear skin side down for 3 to 5 minutes or until most of the fat has rendered and the skin is golden brown, flipping the breast over occasionally. Do not cook breasts on high heat or the skin will shrink a tremendous amount and get chewy instead of crispy.
Add the thyme sprigs and garlic cloves and sauté for 1 minute. Set the thyme, garlic, and duck skin trimmings evenly on top of the breasts with the skin side down so the thyme and garlic can absorb into the meat while roasting. Place in the center rack of the oven and roast for 8 to 10 minutes, or until slightly springy when pressed.
The internal temperature should reach 55°C for medium rare. Transfer to a sheet tray or plate to rest 8 to 10 minutes minutes before slicing.