Bring a large pot of salted water to a rolling boil. Add 8 ounces of rotini and cook according to the package instructions, usually about 8-10 minutes, until al dente. Drain and set aside.

In a large skillet, heat 1 tablespoon of olive oil over medium-high heat. Add the sliced 7 ounces of kielbasa and cook for about 3-5 minutes until golden-brown. Transfer to a plate, leaving drippings in the skillet.

Melt 2 tablespoons of unsalted butter in the same skillet over medium heat. Add 10 ounces of broccoli florets, cover to steam for 5-10 minutes until bright green and crisp-tender.

Sprinkle 1 tablespoon each of cornstarch and flour, ½ teaspoon of kosher salt, and ¼ teaspoon of black pepper over the broccoli. Gradually add 2 cups of whole milk, stirring to combine before bringing it to a simmer. Slowly whisk in 16 ounces of shredded mild cheddar cheese until melted.

Add the cooked rotini and browned kielbasa back into the skillet, stirring well to coat in the cheese sauce. Heat for about 1 minute to warm through.