Melt the butter in a large, heavy-bottomed pot over medium heat. Once foamy, add the onions and sauté until just beginning to brown, about 5 minutes. Add all of the cabbage to the pot and season generously with salt and pepper. Sauté the cabbage, stirring often, until softened and glossy, about 5 minutes more. Add the wine and continue to cook until it has mostly evaporated, again, about 5 minutes. Add the balsamic vinegar and the bay leaf, stir, and cover the cabbage, reducing the heat to low. Cook the cabbage for 45 minutes, setting a timer to remind you to stir it every 15 minutes. After 45 minutes, cut the pears into a medium dice and add them to the pot. Stir and cover, cooking 15 minutes more. Check the seasoning of the dish, adding salt, pepper, or a little more vinegar if necessary. Serve the cabbage hot.