Chop cabbage into strips. Peel carrots and slice them into chunks.
In a dutch oven or heavy-bottomed pot, heat butter over medium heat. Once melted brown brisket on each side. Reduce heat to low.
Add chopped vegetables, maple syrup, soy sauce, and mustard.
Cover pot and simmer until meat is done, approximately 2 hours. You want the brisket to be very tender but still sliceable. When it reaches this point remove it.
Reduce any liquid in the pot until it is a thick sauce. Salt to taste.
Slice brisket and serve alongside vegetables.