Bring 6 quarts of salted water to a boil in a large pot.
Add the pasta and cook until al dente, 6 to 8 minutes or according to packaging.
Drain the pasta and reserve about 2 tablespoons of the pasta water.
Heat olive oil in a large skillet over medium heat and cook sausage until browned for about 3 minutes or until cooked.
Add onion and stir until tender for about 1-2 minutes. Add garlic and saute until lightly browned but do not burn.
Add the optional red pepper flakes and cook for 1 minute. Stir in kale and cook until tender.
Toss in drained cooked pasta and reserved water into the skillet. Stir for about 1 minute.
Add heavy cream and Italian seasoning. Keep stirring for 1 minute.
Add salt/pepper to taste. Remove from heat.
Serve with fresh herbs and cheese