Step 1Toast the mustard seeds in a pot over medium-low heat just until they start to pop, 2 to 3 minutes. Add the cider and water to the pot, and stir in the salt, one cup maple syrup and the bourbon. Add the crushed rosemary sprigs and bring to a boil. Remove from heat and set aside until the brine cools to room temperature.
Step 2Place the turkey in a large non-reactive container and pour over the brine. Place a plate over the turkey to weigh it down so it stays submerged in the brine. Cover and refrigerate overnight.
Step 3The next morning, remove the turkey from the brine and dry with paper towels. Place the turkey, uncovered, on a rack and refrigerate until about an hour before cooking.
Step 4About an hour before cooking, prepare the smoker or grill to cook over low, indirect heat: Set up a drip pan underneath where the turkey will smoke, and fill with water (or the liquid used to soak the wood chips). Shortly before cooking, adjust the heat as needed to maintain a temperature right around 250 degrees and add the soaked chips to start smoking. Melt the butter in a saucepan and stir in the remaining maple syrup; this will be used to baste the turkey as it cooks.
Step 5Baste the turkey and place it (breast-side up) over the drip pan in the prepared smoker. Adjust the heat as needed (add several coals to either side of the grill as needed if using a kettle grill) to keep the smoker between 250 and 300 degrees; replenish the chips as needed to keep smoking for the first hour. Baste the turkey every 20 minutes or so to keep it moist.
Step 6Cook to an internal temperature of 160 degrees in the thickest part of the thigh, 2 to 3 hours (timing will vary depending on the size of the turkey and heat of the smoker). To crisp the skin, open the vents of the grill or smoker to increase the heat, and continue to cook the turkey for 5 to 10 minutes more.
Step 7Remove and set aside 15 to 20 minutes to rest before carving.