Heat the olive oil in a medium (5-quart) soup pot or dutch oven over medium heat. When the oil begins to shimmer, add the onions and sauté until browned, stirring often, about 15 minutes. Add the garlic and sauté for two more minutes.
Add the sausage meat to the pot, breaking it up with a spoon as it cooks. Sauté until all of the sausage is opaque and cooked through, about 5 minutes.
Add the kale to the pot and sauté until wilted and reduced in volume, about two minutes.
Add the tomato paste and stir into the other ingredients to break up any clumps.
Add the cannellini beans and chicken broth, give the pot one more stir, then cover the pot and bring up to a boil.
When the soup begins to boil, turn the heat down to low and let simmer, covered, for 30 minutes.
While the soup is cooking, prepare the pasta according to package instructions.
When the soup and pasta are ready, measure about 2/3 cup of cooked pasta into each serving bowl. Ladle the soup over the pasta and serve hot, garnished with red pepper flakes if desired.