How do you flatten fabric that's been stretched in a hoop for months?

Do all of this before mounting. If you're going to stretch it over stretcher bars or lace it into a frame, the blocking gets you square, and then you can put gentle even tension on during mounting. If you're framing under glass, let the blocked piece rest a day after drying so any remaining moisture doesn't get sealed in. The potato chip shape will mostly relax once it's pinned flat and dry, so don't panic if it looks stubborn mid-process.

Skip the iron contact entirely and use steam. I did a 28 inch wildflower piece last fall with heavy bullion work and I held the iron about half an inch above the back while it was pinned face down on a thick towel, letting the steam do the work. Never touched the fabric once and the knots stayed proud. I left it pinned on a foam board for two full days before it stopped fighting me. Corners were the last thing to settle, so give them extra pins angled outward. Dry pinning alone works too but takes way longer in my expirience.

Back in the day we did this on a spare bed or a big sheet of cork board with rustproof pins and a plant mister, and it still works fine. Mist the back till it's damp but not soaked, pin it out square working from the center outward, and walk away overnight. The knots don't mind a light mist as long as you're not rubbing them. One thing I'd check before you start: if your ground fabric and any variegated floss behave differently when wet, test a corner scrap first. Cheaper to learn that on a snip than on a summer's work.

Update: went with the steam idea, held the iron above the back with it pinned face down on a towel like the wildflower person suggested. It flattened out way better than expected, but now the fabric feels kind of stiff in spots where I got it too steamy. Is that going to soften back up once it fully dries, or did I scorch something? Still haven't mounted it yet.