Preheat Oven: Preheat your oven to 375°F (190°C). Grease a 9×13 inch baking dish and set aside.
Prepare Potatoes: Peel and thinly slice the russet potatoes. Parboil them in a large pot of water for 5-7 minutes, until just tender but not fully cooked. Drain and set aside.
Make Cheese Sauce: In a medium saucepan, melt butter over medium heat. Add flour and whisk continuously for 2 minutes. Gradually whisk in milk and cream, cooking until thickened (5-7 minutes). Remove from heat and stir in cheddar, Gruyère, salt, pepper, and nutmeg until smooth.
Assemble the Dish: Layer half the potato slices in the greased baking dish. Pour half the cheese sauce over them. Repeat with remaining potatoes and cheese sauce.
Top & Bake: Mix Parmesan and breadcrumbs in a small bowl, then sprinkle evenly over the casserole. Bake for 25-30 minutes, or until the top is golden and crispy.
Garnish & Serve: Let the dish cool slightly before serving. Garnish with fresh parsley if desired.
Enjoy the irresistible creamy, cheesy goodness of Delmonico Potatoes!