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1. Allow the Delmonico potatoes to cool completely before transferring them to an airtight container.
2. Store them in the refrigerator for up to 3 days.
3. To reheat, place the desired portion in a microwave-safe dish and heat in the microwave until warmed through, or reheat in the oven at 350°F (175°C) until hot.
4. Start by peeling the russet potatoes using a vegetable peeler to remove the skin.
5. Rinse the peeled potatoes under cold water to remove excess starch.
6. Cut the potatoes into uniform slices, about 1/4 inch thick.
7. Place the sliced potatoes in a large bowl and cover them with cold water to prevent browning.
8. Melt 4 tablespoons of unsalted butter in a medium saucepan over medium heat.
9. Add the finely chopped onion to the melted butter and sauté for about 5 minutes until translucent.
10. Stir in the minced garlic and cook for an additional 1-2 minutes.
11. Pour in the heavy cream and stir to combine, allowing it to come to a gentle simmer.
12. Add the salt, black pepper, and cayenne pepper (if using) to the cream mixture and stir well.
13. Gradually add the shredded cheddar cheese and grated Parmesan cheese to the cream sauce, stirring until melted and smooth.
14. Preheat your oven to 350°F (175°C).
15. In a greased 9×13 inch baking dish, layer the sliced potatoes, slightly overlapping them.
16. Pour a portion of the cheese cream sauce over the first layer of potatoes.
17. Repeat the layering process until all potatoes and sauce are used, finishing with a layer of sauce on top.
18. Sprinkle additional shredded cheddar and Parmesan cheese over the final layer of sauce.
19. Cover the baking dish with aluminum foil to prevent browning.
20. Bake in the preheated oven for 45 minutes.
21. After 45 minutes, remove the foil and continue baking for an additional 15-20 minutes until golden brown and bubbly.
22. Check for doneness by inserting a fork into the potatoes; they should be tender.
23. Once baked, let the Delmonico potatoes sit for about 10-15 minutes before serving.
24. Sprinkle chopped fresh parsley over the top before serving.
25. Cut the potatoes into squares or rectangles for serving and use a spatula to lift portions out of the dish.
26. Serve warm as a side dish to your favorite main course.

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