In a large skillet over medium-high heat, add your sausage. Break it up into chunks (if using bulk raw sausage, if not then spread the slices out into an even layer) and let it cook, undisturbed, for about 3 minutes, or until deeply browned. Give it a stir and cook for another 2 minutes or until cooked through and browned. Transfer to a bowl or plate (leave any oil behind, you want about 2 tbsp worth of oil in the pan, if your pan is dry, add 2 tablespoons olive oil in with the onion below).
Reduce the heat to medium and add the onion and ½ teaspoon salt and cook until softened, about 3 minutes. Stir in the sage, potatoes, apples, and 1 teaspoon salt. Spread into an even layer, cover with a lid (or a baking sheet if you don’t have a lid) and cook for 10 minutes (you should have some nicely browned or charred pieces, if it’s burning a lot, reduce the heat). Give it a stir, then pour in ¼ cup water (the steam will help the potatoes cook through). Cover and cook for 5 more minutes, or until potatoes are cooked through.
Stir the sausage back in and cook for another minute just to warm it up. Remove from heat and top with cheese (if using) and cover with the lid again to let the cheese melt, about 1 minute. Top with any remaining chopped up sage, if you have it.
Meanwhile, add the kale and/or cabbage to a medium bowl and toss with the vinegar and remaining ¼ teaspoon salt. Serve the hash with a side of slaw.