Heat the oil and butter in a sauce pot over medium-high heat. When the butter starts to bubble add the onion and sweat until translucent. Add the garlic and cook for 1 minute. Add the potato, water, cream, and nutmeg to the pan. Bring to a boil, then reduce to a simmer. Cook until the potaoes are tender and remove from the heat. Puree the mixture in a high speed blender or with a hand blender. The sauce should be smooth and a little thin. Season with salt and pepper. Pour 1/3 of the sauce into a large bowl. Divide the remaining sauce between 2-3 large baking pans.