Cut cabbage into sturdy wedges and roast until just tender and slightly caramelized. While the cabbage cooks, whisk together the miso butter sauce to drizzle over the finished cabbage. You'll likely have leftover sauce, which is excellent spooned over boiled potatoes, green beans, or roasted sweet potatoes.
While the cabbage roasts, heat 1 tablespoon olive oil in a medium skillet over medium heat. Add the scallions and saute until a little tender, 2 minutes. Add the butter, miso, ginger, brown sugar, vinegar, and water and whisk until the ingredients melt into a creamy sauce, another minute or 2. Remove from heat.