Sausage, Cauliflower, and Kale Potpie

olive oil

sweet Italian sausage links, casings removed and meat broken into 3/4-inch pieces

medium onions, chopped

chopped fresh rosemary

kosher salt and black pepper

all-purpose flour

low-sodium chicken broth

bunch kale, torn into bite-size pieces (about 10 cups)

white wine vinegar

small head cauliflower (about 2 pounds), cut into florets