Begin by bringing a large pot of salted water to a boil over high heat. Add the penne pasta and cook according to package instructions until al dente, usually around 8-10 minutes. In the last two minutes of cooking, gently stir in the fresh broccoli florets. Once the time is up, drain the pasta and broccoli, then set them aside while you prepare the skillet.

In a large non-stick skillet, melt 2 tablespoons of unsalted butter over medium heat. Once bubbling, add the sliced kielbasa and sauté for about 5 minutes, stirring occasionally, until the sausage is lightly browned and fragrant.

Add the diced yellow onion to the skillet, cooking for 2-3 minutes until it softens and turns translucent. Stir in the minced garlic, and sauté for an additional minute until the garlic is fragrant and golden.

Whisk together 1 tablespoon of cornstarch with 1 cup of whole milk in a separate bowl until smooth. Pour this mixture into the skillet, stirring well. Increase the heat slightly and bring to a gentle simmer, allowing the sauce to thicken for about 3-5 minutes.

Once the sauce has thickened, reduce the heat to low. Gradually stir in 1 cup of shredded mild cheddar cheese, ensuring it melts completely.

Add the drained pasta and broccoli back into the skillet, tossing gently to coat everything with the creamy sauce. Season with salt and pepper to taste.