Kale Rigatoni with Sausage

1.In a large skillet, melt 2 tablespoons butter over medium high heat. Add sausage, cook stirring occasionally, until cooked through about 10 minutes. Transfer to a plate. Put a large pot filled with water on high heat.
2.Melt 2 tablespoons butter on medium in the same pan, add onions and fennel; cook, scraping up any browned bits, until vegetables are well browned, about 15 minutes. To the pot of boiling water add 1 tablespoon salt plus pasta and cook until al dente, about 5 minutes then turn off heat.
3.To skillet add kale, garlic, and 1/2 teaspoon salt; cook, stirring occasionally, until kale is just tender; 3-4 minutes.
4.Add pasta, sausage, 2 tablespoons butter, 1 cup pasta water. Cook over medium, gradually adding enough cooking water to coat pasta, about 3 minutes. Season with salt and pepper to taste and serve immediately.