Bring a large pot of salted water to a boil. Cook the tortellini according to the package directions until al dente. Reserve 2-3 tablespoons of the pasta water before draining.
While the pasta cooks, heat a large skillet over medium heat. Add the Italian sausage and cook for 6-8 minutes, breaking it into bite-sized pieces, until browned and cooked through. Transfer the sausage to a plate, leaving about 1 tablespoon of the drippings in the skillet.
Add the butter to the same skillet. Melt it over medium heat, stirring frequently, until it foams and turns golden brown with toasted brown specks, about 3–5 minutes.
Stir in the minced garlic and chopped sage (if using). Cook for 30 seconds until fragrant.
Add the chopped kale and cook for 2-3 minutes, stirring occasionally, until it has wilted.
Return the cooked sausage to the skillet.
Add the cooked tortellini and gently toss everything together.
Pour in the reserved pasta water, one tablespoon at a time, until the sauce lightly coats the pasta.
Stir in the grated Parmesan cheese and season with salt, black pepper, and red pepper flakes, if desired.
Finish with a squeeze of fresh lemon juice for added brightness, if using.
Serve immediately with extra Parmesan cheese and freshly cracked black pepper.