Put the ribs on a rimmed baking sheet and cover with foil; bake at 300°F for 2 1/2-3 hours, until very tender. (Ribs can be cooked up to this point and then wrapped and refrigerated for up to 3 days.)
Meanwhile, stir together the hoisin sauce, vinegar, honey, garlic, ginger and chilli flakes.
Remove the foil, turn the oven up to 425°F, brush the ribs with sauce and roast for another 10 minutes, until dark and sticky. (Alternatively, finish them on the grill over high heat for a few minutes.) To make the cauliflower rice, heat a drizzle of oil in a large skillet over medium-high heat and cook for 3-4 minutes, just until tender.
Serve with the ribs, sprinkled with sesame seeds.