In a saucepan or in the microwave, melt the margarine. Add the salt, pepper, and flour. Stir until smooth, then gradually add the milk with a wire whisk. The mixture doesn't have to get thick, it does that as it cooks in the oven, you're just mixing it all together uniformly at this point.
Add a layer of sliced potatoes to a thinly greased 13X9 pan. You can use the wrapper on the margarine or butter to lightly grease the pan if you like. It doesn't need much.
Then add 1/2 of the cabbage as the next layer, sprinkle with 1/2 of the onions, and then add 1/2 of the smoked sausage or kielbasa.
Then layer it all again, starting with potatoes, then cabbage, then onions, then smoked sausage, and top it all off with the remaining potatoes.
Sprinkle the cheese on top. Stir up the sauce and pour it over the top of all uniformly.
Cover your pan with aluminum foil tightly and bake in a 350 degree oven for about 1 hour, until potatoes are tender. Then remove the foil and bake another 15 minutes at 400 to brown the top slightly or place under the broiler until lightly browned.