Add the potatoes, onion, mustard powder, garlic powder, parsley, Cajun seasoning, onion soup mix, and chicken broth to a 6-quart (or larger) crock pot.
In a skillet over medium-high heat, sear the sliced sausage on both sides. (Optional but highly recommended for flavor. If short on time, you can skip this step.)
Add the sausage to the crock pot and stir. Place the stick of butter on top.
Cover and cook on low for 6 hours or high for 3 hours, until the potatoes are fork tender.
If using fresh or frozen green beans, add them at this stage so they cook through.
Stir everything together. Add in the drained canned green beans (if using) and the shrimp. Stir again, cover, and cook on high for 15 minutes, until shrimp are pink and cooked through.
Spoon into bowls and serve with crusty bread for soaking up the buttery Cajun broth.