In a large sauté pan with a lid, add the brats in a single layer and cover with stock (enough to just submerge by ¼”.) Bring to a simmer over medium-high heat, then reduce heat to low and cover. Cook 10 minutes, or until the sausages are firm to the touch.

Transfer the brats to a plate or cutting board and pat dry (don’t worry about drying them thoroughly, just enough to get most of the moisture off). Drain the liquid from the pan, wipe the pan clean, and return the pan to the stovetop over medium-low heat. Melt the butter and return the sausages to the pan. Brown the sausages uncovered, about 6 minutes total, rotating every 1-2 mins to brown evenly on all sides.

In a large skillet over medium heat, add 1 tbsp olive oil. Add the onion and sauté until translucent. Add the cabbage and season with 1/4 tsp salt and black pepper to taste. Cook about 5 minutes, or until cabbage is wilted and crisp-tender. Add the apple, red wine vinegar, and parsley and cook 1-2 minutes more. Taste for seasoning and add more salt and pepper if desired.

Serve the brats with the cabbage, mustard, and warmed bread!