Begin by bringing a large pot of salted water to a boil over high heat. Add the penne pasta and cook according to the package instructions until al dente, usually about 8-10 minutes. In the final 2 minutes of cooking, toss in the broccoli florets to blanch them. Drain the pasta and broccoli mixture, setting it aside while you prepare the sauce.

In a large skillet over medium heat, melt 2 tablespoons of unsalted butter until frothy. Add the sliced kielbasa and sauté for about 5 minutes, stirring occasionally, until the sausage is nicely browned and caramelized.

Stir in the diced yellow onion and 1 teaspoon of minced garlic into the skillet with the kielbasa. Cook for an additional 2-3 minutes, until the onion is soft and translucent.

In a small mixing bowl, whisk the cornstarch into 1 ½ cups of whole milk until smooth. Carefully pour this mixture into the skillet, stirring constantly. Bring the sauce to a gentle simmer over medium heat until it thickens within 3-5 minutes.

Reduce the heat to low and slowly add 1 ½ cups of shredded mild cheddar cheese, stirring continuously until melted.

Finally, add the drained pasta and broccoli back into the skillet, tossing everything together until well-coated with the creamy sauce. Season with salt and pepper to taste and let it warm through before serving.