In small bowl, combine 2½ tsp of the salt, ½ tsp of the pepper, thyme and brown sugar. Spread ⅔ of mixture over fat cap and remaining mixture over rest of pork. Arrange on rack set over baking sheet and refrigerate, uncovered, for 24 to 36 hours.
Preheat oven to 300°F.
In large Dutch oven or large heavy-bottom pot over medium heat, cook bacon until lightly crisp. Remove to paper-towel-lined bowl and set aside. Add onion to pot and cook, stirring, until translucent, about 6 minutes. Add cabbage, apple cider, 1 cup water, the remaining 1½ tsp salt and ¼ tsp pepper, stirring. Bring to a boil, cover and cook 25 minutes. Remove lid and cook until scant liquid remains, about 10 to 12 minutes.
Nestle pork over cabbage mixture and roast, uncovered, until a thermometer inserted in centre registers 130°F, about 1½–2 hours. Remove from oven and increase oven temperature to 450°F. Return roast to oven and cook until thermometer registers 145°F and top is golden, about 10 to 12 minutes longer. Remove pork to cutting board and let rest.
Place pot over medium heat; add cream and bring to a boil. Cook, stirring, and let mixture reduce until thickened and cabbage is coated in cream.
Roughly chop reserved bacon and add to pot along with mustard seeds, stirring to combine. Carve pork and serve with cabbage and your favourite mustard.