Slow-Cooker Brat and Sauerkraut Soup

uncooked bratwurst, cut into 1/2-inch-thick slices

tiny new red potatoes, cut into quarters

small onions, cut into 1/4-inch wedges or coarsely chopped

sliced celery

garlic, minced

bay leaves

14 1/2 - ounce can Bavarian-style sauerkraut

14 1/2 - ounce cans lower-sodium beef broth

spicy brown mustard

cider vinegar

paprika

fennel seeds, crushed

caraway seeds

Sour cream (optional)