Use preground fennel and coriander seeds if you’re short on time; you’ll lose the texture the crushed seeds provide, but you’ll get the same great flavor. Substitute 2 teaspoons each of the ground seeds for 1 tablespoon whole seeds.
Place fennel seeds and coriander seeds in a spice grinder; pulse until coarsely ground. Stir together ground seeds, pepper, and 1 teaspoon salt in a small bowl; sprinkle evenly over pork.
Heat oil in a large nonstick skillet over medium-high. Add pork to skillet; cook, turning to brown on all sides, about 8 minutes. Transfer pork to a 5- to 6-quart slow cooker; reserve drippings in skillet.
Add chicken stock, wine, and soy sauce to reserved drippings in skillet; bring to a boil over medium-high, stirring to loosen browned bits from bottom of skillet. Pour mixture over pork. Add sweet potatoes, Yukon Gold potatoes, shallots, and thyme sprigs to slow cooker. Cover and cook on low until pork and vegetables are tender, about 8 hours.
Transfer pork to a cutting board, and let rest 5 minutes. Transfer vegetables to a platter; reserve cooking liquid in slow cooker. Discard thyme sprigs. Break pork into large pieces; discard bone. Place pork on platter with vegetables; sprinkle with remaining 1/2 teaspoon salt. Garnish with thyme leaves, if desired. Serve with reserved cooking liquid.