Best Au Gratin Potatoes

Ingredients

• 2 pounds Yukon Gold potatoes, peeled and thinly sliced
• 1 teaspoon salt
• 1/2 teaspoon black pepper
• 1/4 teaspoon nutmeg
• 4 tablespoons unsalted butter
• 1/4 cup all-purpose flour
• 3 cups whole milk
• 1/2 teaspoon garlic powder
• 1/2 teaspoon onion powder
• 1 1/2 cups shredded Gruyère cheese
• 1/2 cup grated Parmesan cheese
• Fresh parsley for garnish (optional)