Heat the oven to 400°F.
Spread the bacon in one layer on a large uncoated stainless-steel half sheet pan. Bake for 10 to 20 minutes, depending on thickness, until crisp but not burned.
Move the bacon to a plate and leave all rendered fat on the pan.
Add the cauliflower, sea salt, and optional black pepper. Toss with two spoons until every floret is glossy, then spread into one layer.
Roast for about 22 minutes, until the stem pieces are tender and the exposed edges are deeply golden brown.
Add the scallion whites and optional aged cheese. Reduce the oven to 200°F and bake for 5 minutes, just until the whites soften and cheese melts.
Chop the cooled bacon. Transfer the cauliflower to a platter or plates and top with bacon, scallion greens, and plenty of fresh thyme leaves.