Step 1: Heat the oven and prep the dish

• Set the oven to 375 degrees F.
• Grease a 9 by 13 inch casserole with butter or olive oil.
• Warm the cream and milk together until just steamy, not boiling. Warm dairy integrates faster into the roux and reduces lumps.

Tip: A metal or enameled dish promotes better browning than thick glass.

Step 2: Slice the potatoes with precision

• Peel the potatoes if you prefer. Yukon Gold skins are tender, so peeling is optional.
• Using a mandoline, slice to 1/8 inch. If using a knife, stay as consistent as possible.
• Place slices in cool water to prevent browning while you work, then drain and pat very dry. Excess water can dilute the sauce.

Data note: Moving from 1/4 inch slices to 1/8 inch reduces bake time by roughly 25 to 30 minutes and improves tenderness uniformity.

Step 3: Build a quick, stabilized sauce

• Melt 4 tablespoons butter in a medium saucepan over medium heat.
• Sauté minced shallot and garlic for 2 to 3 minutes until soft and fragrant, not browned.
• Whisk in 3 tablespoons flour to make a smooth paste. Cook 1 minute to remove raw flour taste.
• Slowly whisk in the warm milk and cream, then add thyme, bay, nutmeg, salt, and pepper.
• Simmer 3 to 4 minutes until the sauce lightly coats a spoon.

Pro move: Pull the bay leaf before layering. Taste and adjust salt now so the sauce is flavorful on its own.

Step 4: Assemble in layers that melt together

• Arrange a snug layer of scalloped potatoes on the bottom of the dish.
• Spoon on about 1/3 of the sauce.
• Sprinkle a mix of Gruyere and cheddar over the sauce.
• Repeat for two more layers, finishing with sauce and cheese. Dust Parmesan on top if using.

Tip: Keep the top layer neat and slightly shingled for an elegant look once browned.

Step 5: Bake covered first

• Cover tightly with foil.
• Bake for 40 to 45 minutes. The steam inside will gently cook the scalloped potatoes and set the sauce without breaking it.

Check: You should be able to slide a knife in with slight resistance at this point.

Step 6: Uncover and brown to finish

• Remove the foil.
• Bake 30 to 35 minutes until bubbling, golden, and tender throughout. The top should be blistered in spots and caramelized at the edges.

If the top browns too fast, tent loosely with foil; if too pale at the end, move to the upper rack for 5 more minutes.

Step 7: Rest for the cleanest slices

• Let the casserole sit 15 minutes on a rack. Starches finish setting and the sauce thickens to sliceable creaminess