Preheat your oven to 375°F (190°C) and grease a 9×13 baking dish. Create a cheese sauce by melting 4 tablespoons of butter in a saucepan, whisking in ¼ cup of flour, then slowly adding 2 cups of milk or cream while stirring until the mixture thickens. Layer half of the sliced potatoes in the baking dish, followed by half of the sausage and onion slices. Pour half of the cheese sauce over the top. Repeat the layers with the remaining potatoes, sausage, onion, and cheese sauce. Top with the remaining shredded cheddar cheese. Cover the dish with foil and bake for 45 minutes. Remove the foil and bake for an additional 15 minutes, or until the potatoes are tender and the cheese is bubbly and golden brown. Let the gratin rest for a few minutes before serving, then garnish with fresh herbs if desired.