Preheat the oven to 375F/190C.
Cook pasta in a large pot filled with salted water 3-4 minutes less than the package directs.
While pasta is cooking make the cheese sauce. Melt butter in a small saucepan over medium heat, blend in flour with a wire whisk.
Continue cooking for 2-3 minutes while stirring the entire time, then add milk and stir.
Cook the sauce for 5 minutes, it will gradually thicken, continue stirring with a wire whisk to avoid lumps.
When the sauce is the desired consistency take it off the heat and stir in both cheeses, reserve a combined ¼ cup of cheese.
The cheese should melt and completely incorporate into the sauce. Use your wire whisk to ensure the sauce is smooth and lump-free. (Don't walk away from the stove the entire time the cheese sauce is cooking as it can burn quickly and/or will go lumpy if left unattended.)
When your pasta is cooked drain but reserve ¼ cup of starchy water the pasta was cooking in.
Return the drained pasta into the same pot and add water and cheese sauce, gently toss to coat the pasta. It should be quite runny.
Mix in sliced kielbasa and transfer to a deep baking dish, sprinkle with the remaining ¼ cup of cheese.
Bake in the preheated oven for 30 minutes. The top should be golden by the end of the cooking time. If the pasta still looks pale at the end of the cooking time set your oven to broil and let the top crisp up for 2-3 minutes.
Serve with a simple green salad.