Melt 2 tablespoons unsalted butter in a large cast iron or soup pot over low heat. Add sliced celery, onions and sweet apples, stirring to coat in the butter. Season with a pinch of kosher salt and several turns of freshly ground black pepper. Cook on low, stirring often, until the vegetables are soft (about 10 minutes). Add celery root cubes and turn them in the butter. Add bay leaf, thyme sprig and chicken stock and bring the mixture to a boil. Reduce heat to low and gently simmer until the celery root is soft (about 30 - 35 minutes). You can test the doneness of the celery root with a fork. Spear one of cubes with a fork, then gently press it against the side of the pot. If you can easily mash the cube, it is done.