Heat the butter in a large cast iron pot or dutch oven over medium-high heat. Once the butter starts to shimmer, add the sliced onion and saute, frequently stirring until just soft, about 3 minutes. If using sausage, add it together with the onion. Add the rest of the ingredients, stir and bring to boil. Lower the heat to medium-low, cover and simmer until the potatoes are fork-tender, about 15 minutes, stirring occasionally.