01 - Set the oven temperature to 375°F.
02 - Melt butter in a large ovenproof pot over medium heat and cook the sliced onion until softened, about 4 minutes.
03 - Stir in the apples and continue cooking for 2 minutes.
04 - Add the sliced red cabbage, apple cider vinegar, brown sugar, allspice, cinnamon, salt, black pepper, and water to the pot; mix well.
05 - Cover and cook over medium-low heat for 25 minutes, stirring occasionally until the cabbage is wilted and slightly tender.
06 - In a bowl, mix oats, whole wheat flour, brown sugar, salt, and thyme. Add diced butter and rub with fingertips until mixture forms coarse crumbs.
07 - Remove the pot lid, taste the cabbage mixture, and adjust seasoning if needed.
08 - Evenly sprinkle the oat topping over the cabbage mixture in the pot.
09 - Place the uncovered pot into the oven and bake for 35 to 40 minutes, until the topping is golden and crisp.
10 - Allow the dish to stand for 5 minutes before serving.