Preheat oven to 325°F (160°C).
Season pork belly with salt and pepper. Place in a roasting pan and cook for 2-2.5 hours, or until tender and crispy.
While pork belly is cooking, make the soup. In a large pot, combine celeriac, onion, apples, garlic, white wine, butter, buttermilk, salt, and sugar. Bring to a boil, then reduce heat and simmer for 30 minutes, or until celeriac is tender.
Blend the soup until smooth. Season to taste.
In a small bowl, whisk together pecans, olive oil, mustard, apple cider vinegar, and salt. Set aside.
In a large skillet, heat olive oil over medium heat. Add kale and onion and cook until wilted and softened, about 5-7 minutes.
Slice pork belly into bite-sized pieces.
Serve soup topped with pork belly and kale mixture. Drizzle with pecan vinaigrette.