In a dry skillet, combine the fennel seeds, coriander seeds, and peppercorns. Place over medium high heat, swirling the spices in the pan regularly to ensure even browning. Toast the spices until they darken in colour and begin to lightly smoke. Remove from the heat and transfer the spices to a separate plate to cool.
Once cooled, grind the toasted spices in a spice grinder or blender until finely powdered. Transfer to a small bowl and add the kosher salt, garlic powder, onion powder, and allspice. Stir to combine. Remove 2 tablespoons of the spice rub and set aside to use when roasting the parsnips.
To prepare the pork leg, use a sharp knife to remove the layer of skin on the bottom of the leg. Trim any large pockets of fat on the bottom as needed. Rub a generous amount of the spice rub over the exposed meat on the bottom and front end of the pork leg. Do not rub over the skin.
Place a rack in the bottom of a large roasting pan. Arrange the pork leg on the rack with the skin side up. Pat the skin dry. Use a sharp knife to score the skin on the top and down the sides of the leg. Make sure to cut through the skin but not into the flesh. Refrigerate the pork leg overnight uncovered to allow the skin to dry out. This will help to crisp the skin while roasting.
Preheat the oven to 450°F. Rub the skin of the pork with olive oil and season generously with kosher salt. Pour 2 cups of water into the bottom of the roasting pan and then place in the oven. Roast the pork uncovered for about 45 to 50 minutes. Turn down the heat to 325°F and continue roasting for another 3 1/2 to 4 hours until it reaches an internal temperature of 145°F. Check the roasting pan occasionally to see if the water has evaporated, adding more if needed.
When the pork has reached 145°F, turn the temperature back up to 425°F. Continue roasting for another 30 to 45 minutes until the skin is blistered and crispy and the pork has reached an internal temperature of 155°F. Remove from the oven and let rest for about 20 to 30 minutes before carving. Pour off the drippings from the roasting pan. Skim and discard the fat from the drippings and set aside for the glaze.
Prepare the parsnips while the pork is roasting. Peel the parsnips. Depending on size, split them in half lengthwise or quarter them lengthwise. Transfer to a baking sheet and drizzle with olive oil. Season with the reserved spice rub, tossing to evenly coat the parsnips. Begin roasting the parsnips during the final stage of the pork when the oven temperature is raised to 425°F. Prepare the glaze while the parsnips are roasting.
Roast the parsnips for 25 to 30 minutes until they are almost tender. Drizzle with a few tablespoons of the glaze and return to the oven. Roast until the parsnips are fork tender and beginning to char around the edges.
To make the glaze, melt the butter in a saucepan over medium heat. Add the honey, apple cider vinegar, Dijon and whole grain mustards, kosher salt, pepper, and allspice. Whisk to combine and then bring to a simmer. Set aside 2 to 3 tablespoons for the parsnips. Measure the drippings, adding water if needed to make about 1 cup. Add the drippings to the remaining glaze and bring to a simmer. Taste and adjust the seasoning as needed.
Use a sharp knife to carve the pork leg into slices. Remove and break apart the skin to serve with the meat. Arrange the pork and parsnips on a platter and serve with the tangy mustard glaze. Enjoy!