2 tablespoons olive oil
1/2 onion, diced
salt to taste
8 ounces spicy Italian sausages, casings removed
3 1/2 cups low-sodium chicken broth, divided, or as needed
1 1/4 cups orecchiette pasta, or more to taste
1/2 cup roughly chopped arugula, or to taste
1/4 cup finely grated Parmigian cheese