In a large pot or Dutch oven, heat olive oil over medium-high heat. Add the Italian sausage, breaking it into small crumbles with a wooden spoon. Cook until deeply browned and cooked through (internal temperature of 160°F/71°C), about 6-8 minutes. Transfer to a paper towel-lined plate, leaving about 1 tablespoon of fat in the pot.

Reduce heat to medium. Add the diced onion to the pot and cook until softened, about 3-4 minutes. Stir in the minced garlic and red pepper flakes, cooking for another minute until fragrant.

Add the ditalini pasta directly to the pot, stirring constantly for 1-2 minutes to lightly toast it. Pour in the chicken broth, scrape up any browned bits from the bottom, and bring to a boil. Reduce heat to a simmer and cook, uncovered, stirring occasionally, until the pasta is just shy of al dente, about 8-10 minutes (it will finish cooking in the next step).

Stir in the heavy cream and chopped kale. Return the browned sausage to the pot. Continue to simmer, stirring often, until the kale is wilted and tender, the pasta is fully cooked, and the sauce has thickened slightly, about 5-7 minutes.

Remove the pot from heat. Stir in the grated Parmesan cheese until melted and well combined. Season generously with salt and black pepper to taste. Let the dish rest for 5 minutes off heat to thicken slightly.