In a large, enameled cast-iron casserole, melt the butter in the bacon fat. Add the Brussels sprouts in batches and cook over high heat, stirring, until softened but still bright green, about 8 minutes. Season with salt and pepper. Add the apples and thyme; cook, stirring, until the apples are warmed through. Transfer the slaw to a platter, scatter the bacon on top and serve.