Combine the salt and the water in a large resealable plastic bag (or a large bowl) and stir until the salt crystals dissolve.
Add the peppercorns, thyme, garlic, bay leaf, coriander, if using, and duck leg quarters. Seal the bag, put in a pan or bowl (to contain any potential leaks) and refrigerate for 24 hours.
Drain the duck leg quarters (discard the brine) and rinse under cold running water. Pat dry with paper towels. Prick the skin all over with a darning needle or sharp fork, being careful not to nick the meat. (It helps if you go in at an angle.) This creates channels for the fat to escape, making for crispier skin.
When ready to cook, start the Traeger on Smoke with the lid open until the fire is established (4 to 5 minutes). Set the temperature to 400F and preheat, lid closed, for 10 to 15 minutes.
Meanwhile add enough olive oil to a large cast iron skillet or roasting pan to film the bottom. Arrange the duck leg quarters in the skillet or roasting pan in a single layer, skin-side down.
Put the skillet or roasting pan on the grill grate. Roast the duck for 30 minutes, or until the duck fat begins to render. Reduce the temperature to 300F. Turn the duck legs so they are skin-side up. Cover the skillet or roasting pan tightly with foil.
Continue to roast the duck for 2 hours. Uncover the duck and roast for an additional hour, or until the skin is crisp and golden brown. Remove the duck, shred, and serve immediately. (Alternatively, you can refrigerate the duck for up to a week. Re-crisp the skin by grilling the duck, skin-side down, in a hot cast iron skillet or on your Traeger.) Serve with brown butter french fries.
*Strain the remaining duck fat through cheesecloth or a fine-mesh kitchen strainer and transfer to a covered container; refrigerate for up to 6 months. Use the flavorful fat to saut potatoes or sturdy greens.