Heat a large cast iron (or other oven safe) skillet over medium-high heat and add the olive oil and sausage slices.
Cook for a few minutes, then mix in the onion. Continue cooking until the onions begin to soften and the sausage slices are browned and slightly crisp.
In a large bowl, mix together the tomato sauce, brown sugar, salt, ground mustard, apple cider vinegar, and water until combined.
Pour the mixture into the cast iron skillet along with the beans and stir to combine. Cover the pan with foil or an oven-safe lid and bake covered for 2 hours. After 2 hours, remove the lid or foil, and bake for another 30-40 minutes, or until thickened.