In a 5- to 6-quart pan over medium-high heat, stir sausage often until lightly browned, 6 to 8 minutes. Spoon out and discard all but 1 tablespoon fat from pan.
Add onion and garlic to pan; stir often until limp, 3 minutes.
Increase heat to high. Add broth, beer, potatoes, cabbage, carrots, coriander, allspice, and peppercorns (wrap and tie spices in cheesecloth if desired). Cover and bring to a boil; reduce heat and simmer until potatoes are tender to bite, 10 to 15 minutes. Discard spices if wrapped.
Stir in parsley. Ladle soup into bowls. Serve mustard alongside to add to taste.