Begin by bringing a large pot of salted water to a boil. Add the 8 ounces of penne pasta and cook it according to the package instructions. In the final 2 minutes of cooking, carefully stir in the 2 cups of quartered broccoli florets. Once everything is tender and al dente, drain the pasta and broccoli, and set them aside for later.

In a large skillet, melt 2 tablespoons of unsalted butter over medium heat. Once melted and bubbling, add the 13.9 ounces of sliced kielbasa sausage to the skillet. Cook for about 5 minutes, stirring occasionally, until the kielbasa is golden brown and lightly crisped.

Add the 1 small diced yellow onion and 1 teaspoon of minced garlic to the skillet with the kielbasa. Sauté them together for an additional 2 to 3 minutes, until the onion becomes translucent and fragrant.

In a small bowl, whisk together the 1 ½ cups of whole milk and a tablespoon of cornstarch until smooth. Pour this mixture into the skillet, scraping up any browned bits from the bottom. Bring the sauce to a simmer, stirring continuously for about 3 to 5 minutes until it thickens nicely.

Lower the heat to low and gradually add in the 1 ½ cups of shredded mild cheddar cheese to the thickened sauce. Stir gently until the cheese melts completely.

Add the cooked penne pasta and broccoli to the skillet, tossing everything together gently until the pasta is well coated in the sauce. Season with salt and pepper to taste, allowing the dish to heat through for a minute before serving.