Heat oven to 140C/120C fan/gas 1. Bake the potatoes for 2 hrs until soft to the middle (alongside the pork is fine).
Toss the potatoes with the olive oil, most of the spring onions, thyme and plenty of seasoning in a slightly-too-small roasting tin. Use the back of a fish slice or similar to squash the potatoes into a flat-ish compact cake. Chill until ready to finish, for up to a day.
Heat oven to 220C/200C fan/gas 7 (when the pork comes out). Drizzle the spuds with a little more oil and roast for 30-40 mins until golden and crisp. Mix the salad cream and soured cream, drizzle over, then scatter with remaining spring onions and more seasoning, and serve.