Add 4 cups of salted water to a pot and bring it to a boil. Once it starts boiling, add the pasta and cool Al dente. Drain the pasta and set it aside.
Over medium heat, add butter to a large pan and melt it. Then add onion and saute for 5 minutes, and add the garlic and red pepper flakes. Cook for 1 minute.
Whisk in the flour and coof for an additional minute.
Next, slowly alternate incorporating the heavy cream and 2/3 cup of milk. Whisk it often until it's smooth and the sauce starts to thicken. This typically takes about 5 minutes.
While the sauce is thickening, mix the ricotta, heavy cream, remaining milk, parmesan, basil, salt, pepper, lemon zest, and juice all in a small bowl. Mix well with a whisk to combine.
Add the mixture to the sauce and whisk to combine. Reduce heat to a simmer and cook for 5 minutes.
Add in the kale and stir it in. Cook until the kale starts to wilt.
Stir in the pasta and top with more parmesan garnish with basil and lemon zest before enjoying.