Place a large stockpot over medium heat. Cut the bacon strips into 4 pieces each. Separate the bacon pieces and place in the pot. Stir and sizzle until the bacon is cooked, but not fully crispy.
Meanwhile, peel the onion and cut it into 8 wedges. Then cut the cabbage in half and remove the core. Cut the cabbage into 12 wedges. Once the bacon is mostly cooked, add the onion to the pot and sauté for 3-4 minutes to soften.
Add the cabbage, chicken broth, and mustard seeds. Stir to move the cabbage to the bottom of the pot and bring the bacon upward. Cover and simmer for 15-20 minutes, until the thickest parts of the cabbage are tender. Stir once or twice while cooking. Taste, then salt and pepper as needed. Serve with whole grain mustard on the side, if desired.