In a wide skillet, melt the butter over medium heat.
Add onion and sauté until it begins to soften.
Add cabbage and apple and cook, stirring often, until cabbage is tender. Season well with salt and freshly ground black pepper.
Pour stock over, stir, and bring to a boil. Reduce heat, cover, and simmer while you cook the sausage.
In another pan, cook the sausage over medium low heat until cooked through, turning often and cutting larger pieces in half until meat shows no pink inside. Drain cooked sausage on paper towels and then add to vegetable mixture.
Return soup to a boil and add spinach a handful at a time, stirring until wilted.