Heat 2 Tbsp of olive oil in a cast iron skillet. Add diced onion and garlic. Sauté on low-medium heat, stirring until softened.
Drain the beans. Add to the skillet with salt and pepper. Smash some of the beans to add flavor. Stir in the milk and simmer on low-medium until thickened.
While the bean mixture is simmering, cook the optional sausage crumbles until browned. Add balsamic vinegar to the sausage in the pan, stir, and simmer until caramelized.
