Preheat oven to 350°F and adjust rack to middle position.
Heat olive oil in cast iron pan and add the diced kielbasa and onions in an even layer, cook until browned. Remove from skillet and set aside.
Pour off the fat and strain it, then return it to the same skillet and set it to medium-low. Add the butter and the flour and whisk it together until a thick paste forms, about 1 minute.
Add the milk by the quarter-cup, whisking well after each addition. Cook until the sauce has thickened then add the chicken stock in the same way. Add 4 oz of TRF Garlic Kraut Mustard and sauerkraut and mix until combined.
Remove from the heat and add 2 oz of gruyere and 2 oz of cheddar to the sauce and season with salt and pepper.
Spread a ladleful of sauce onto the bottom of a 9x13 baking dish, then top with a layer of sliced potatoes, and then a layer of kielbasa. Cover with sauce and continue until all of the ingredients are used. Finish with a layer of potatoes and then finally sauce on top.
Baked, covered with foil, for 40 minutes.
Remove from oven and add the rest of the cheese.
Bake for an additional 30 minutes, or until potatoes are tender and the cheese is slightly browned.