Heat 1 tbsp oil in a large skillet. Cook sausage until no longer pink and starting to brown. Remove sausage to a paper towel lined plate. In same skillet over medium heat, add remaining tbsp oil and sauté onion for 3-4 minutes, add garlic and continue to cook for 2 minutes. Stir in kale, broth, Italian seasoning, pepper and chili flakes. Allow to simmer uncovered, stirring occasionally until liquid is absorbed and kale is tender, about 15 minutes. There should be no visible liquid left. Remove to a large bowl and allow to cool to room temperature.