Heat oil in a large saucepan over medium-high heat and add garlic, onion and red chili flakes. Cook until golden brown
Add sausage, and cook, breaking it up into small pieces until browned
Add kale, season with salt and pepper, and cook until wilted
Add half and half and bring to a simmer; cook, stirring occasionally until reduced by a third
Stir in Parmesan, juice and zest, nutmeg, and season with salt and pepper; set aside and keep warm
Cook penne per package instructions. Reserving 1/2 cup cooking water when draining
Combine pasta and reserved sauce (adding reserved pasta water if desired) and cook about 5 minutes more. Garnished with parsley