Ingredients
Equipment
Method
Prep and heat
- Preheat oven to 375F, then butter a 9x13-inch baking dish.
- Arrange the sliced potatoes and onion so they’re ready to layer.
Make the creamy sauce
- Melt butter, then cook minced garlic for 30 seconds.
- Whisk in all-purpose flour and cook for 1 minute.
- Gradually whisk in whole milk and heavy cream, then simmer until slightly thickened.
- Season the sauce with salt, black pepper, and fresh thyme leaves, then keep it warm.
Layer and bake
- Layer potatoes and onion in the dish in three layers, pouring sauce over each layer.
- Cover with foil, then bake for 1 hour at 375F.
- Uncover, sprinkle evenly with grated Parmesan, then bake 25-30 minutes at 375F until golden and fork-tender.
Rest and serve
- Rest for 15 minutes before serving so the layers set and slice cleanly.
Notes
For the cleanest layers, keep potato slices close to 1/8 inch and spread each layer evenly before pouring sauce over it. Store leftovers in an airtight container in the refrigerator for up to 4 days; reheat covered in a 350F oven until hot. Freeze leftovers up to 2 months for best texture. For a lighter option, use 2% milk instead of whole milk (cream remains for richness).
