Preheat your oven to 375 degrees F. Lightly grease a 9x13-inch baking dish and set it aside.

Place the sliced potatoes in a large pot and cover with water. Bring to a boil, then reduce the heat and maintain a low boil for 4 to 6 minutes. The potatoes should be tender but still firm enough to hold their shape. Drain the water and immediately rinse the potatoes under cold water to stop the cooking process.

Heat a large skillet over medium-high heat and melt 3 tablespoons of the butter. Add the diced onions and sauté for 3 to 4 minutes until they begin to soften. Stir in the sliced cabbage and cook for another 4 to 5 minutes until the cabbage is tender. Remove the skillet from the heat.

In a large pot (you can reuse the potato pot), melt the remaining 2 tablespoons of butter over medium heat. Whisk in the flour and cook for 1 to 2 minutes to remove the raw flour taste. While whisking constantly, slowly pour in the milk and heavy cream. Let the mixture simmer for 3 to 4 minutes until the sauce thickens.

Remove the pot completely from the heat. Stir in the shredded white cheddar, sour cream, dried mustard, and salt and pepper to taste. Keep stirring until the cheese is completely melted and the sauce is smooth.

Pour the finished cheese sauce directly into the skillet with the cooked cabbage and onions, stirring well to combine.

Spoon one-third of the creamy cabbage mixture into the bottom of your prepared 9x13 baking dish, spreading it evenly. Arrange half of the cooked potato slices on top. Add another third of the cabbage mixture, followed by the remaining potatoes. Finish with a final layer of the remaining cabbage mixture.

Scatter the French fried onions evenly over the top of the casserole. Cover the baking dish tightly with aluminum foil.

Bake in the preheated oven for 40 minutes. Carefully remove the foil and bake for an additional 10 to 15 minutes, or until the edges are bubbling and the top is golden brown. Serve warm.