Using a sharp paring knife, score the skin of the duck breasts at 1/3-inch (8 mm) intervals crosswise. Score it again at about 1/2 inch (1 cm) intervals lengthwise. Be careful not to penetrate the meat. Lightly season both sides of the duck breast with salt. Set the breasts skin-side up on a plate, uncovered, and let rest in the refrigerator for 12 to 24 hours. Remove from the refrigerator about 30 minutes prior to smoking to bring them close to room temperature. Pat dry with a paper towel.
Set the smoke chips in the donabe. Set up the bottom and middle grates with the duck breasts skin-side up on each. The thicker pieces should go on the bottom tier. Set the donabe, uncovered, over high heat. Wait until the smoke chips start to release smoke, about 7 to 8 minutes. Cover, and fill the reservoir in the rim of the base a little over half full with water. Smoke for 9 to 10 minutes over high heat, and then turn off the heat. Let rest undisturbed for 20 minutes.
Meanwhile, make the sauce. Whisk together the crème fraîche and wasabi paste in a medium bowl until smooth. Add the usukuchi shoyu and green onion and stir. Gradually stir in the lemon juice. Adjust the seasoning with a pinch of salt, if desired.
Transfer the duck breast to a cutting board and slice crosswise along the scored lines (or thinner if you like). Serve with the dipping sauce on the side.