Preheat oven to 375°F and set out a large oven-safe skillet.
In a medium saucepan over low heat, melt butter and cook onion for 3-5 minutes, until softened.
Add flour, salt, and pepper. Cook for 1 minute, stirring constantly.
Slowly whisk in milk. Cook for about 5 minutes, until thickened.
Stir in cheese until melted and smooth. Remove from heat.
Arrange sliced potatoes in the skillet in an even layer, fanning them slightly.
Nestle the kielbasa slices between the potatoes.
Pour the cheese sauce evenly over the top.
Cover tightly with foil and bake for 60 minutes, or until potatoes are fork-tender.
Remove foil and broil for 2-5 minutes, until the top is golden and bubbly.
Garnish with parsley and serve hot.