• Spray a casserole dish or 9x13 pan with nonstick cooking spray and set aside. Preheat oven to 425° F.
• Peel potatoes and rinse. Slice into 1/4-inch pieces and set aside.
  

for the cream sauce

• In a deep skillet, melt butter over medium heat. Saute onions until lightly browned and fragrant, about 5 minutes.
  
• Sprinkle in flour and whisk until a smooth paste is created. Slowly whisk in milk and bring to simmer.
• Add in salt, black pepper, and nutmeg while waiting for the sauce to bubble. The sauce should thicken once it starts bubbling.
• Remove from heat and stir in heavy cream. Taste and adjust seasonings. Once you get the sauce to taste how you like, add in a little extra salt.

for assembly

• Spread about 1/3 cup cream sauce into the casserole dish. Add about half the potatoes, being sure to separate from each other as best as possible.
  
• Season potatoes with salt and pepper. Top with half the remaining cream sauce and half the cheddar cheese.
• Top with remaining potatoes, sauce and cheese.
• Bake for 1 hour uncovered. Check after 30-40 minutes to ensure the top isn't getting too brown. If it is, cover with foil for remaining baking time.
  
• Let rest for 10-15 minutes before serving. Garnish with fresh herbs like chopped chives or parsley.