In a small dry skillet over medium-heat high heat, add the fennel seeds and peppercorns. Tossing often, toast until the spices are very fragrant, about 3 minutes. Allow the spices to cool completely and then grind in a spice grinder or crush in a mortar and pestle. Stir in 2 teaspoons of the salt.
Rub the spice mixture all over the turkey. To ensure a perfectly crisp, browned bird, refrigerate the turkey uncovered for at least 8 hours or overnight to allow the skin to dry out.
In a small saucepot over medium-high heat, add the Pummelo zest, 1 cup of the juice, honey, rosemary, and a pinch of salt. Stirring often, gently simmer the liquid until it’s reduced by about half. Whisk in the butter and then set aside 1/2 cup of the glaze.
Preheat the oven to 450°F and get out a large roasting pan.
Scatter the fennel and leeks on the bottom of the roasting pan and pour in the remaining Pummelo juice, wine, and stock. Season the veggies with a pinch of salt and pepper.
Gently pat the turkey completely dry with paper towels and then place it on top of the fennel and leeks. Season the cavity generously with salt and pepper and tuck any remaining Pummelo segments, fennel stems, leek stalks, and rosemary inside. Tuck the wings under the bird, tie the legs together with twine, and then thoroughly brush the turkey all over with the 1/2 cup glaze and season with the remaining 1 teaspoon of salt.
Slide the pan into the oven and roast for 20 minutes. Reduce the oven’s heat to 350°F and continue roasting, basting the turkey with the glaze and the drippings in the bottom of the pan about every 30 minutes. Tent the breast or legs with foil if they begin to get too dark. Roast until the juices run clear when you cut between the leg and the thigh, about 2 1/2 to 3 hours.
When the turkey is done, immediately baste it one more time with the glaze and then allow it to rest for 15 minutes. Strain the liquid left in the roasting pan to a saucepot over medium-high heat and bring to a boil. Reduce the heat to a simmer and cook until slightly thickened and reduced, about 10 minutes. Whisk any remaining glaze into the gravy and season to taste with salt if necessary.
Transfer the turkey to a cutting board and carve. Platter the turkey pieces, drizzle with some of the gravy, and garnish with Pummelo wedges and rosemary sprigs. Serve the extra gravy alongside.
