2½ lb (1.1 kg) Yukon Gold potatoes
2 tablespoons butter
1 medium onion, finely diced
4 garlic cloves, minced
2 tablespoons all-purpose flour
1½ cups whole milk
1 cup heavy cream
½ cup grated Parmesan cheese
2 tablespoons chopped fresh parsley, divided
1 teaspoon dried thyme
1 teaspoon salt, or to taste
½ teaspoon black pepper
½ cup shredded mozzarella, optional, for topping
Instructions
Prepare the oven and baking dish.
Preheat the oven to 375°F (190°C). Lightly butter a 9 × 13-inch baking dish or a similar 3-quart casserole dish.
Slice the potatoes.
Scrub the potatoes well. Leave the thin skins on to match the picture, or peel them if preferred. Cut into even slices approximately ¼ inch thick. Cut larger slices in half so the pieces are similar in size.
Cook the onion.
Melt the butter in a saucepan over medium heat. Add the diced onion and cook for 6–8 minutes, stirring occasionally, until softened and lightly golden.
Add the garlic and flour.
Stir in the minced garlic and cook for 30 seconds. Sprinkle in the flour and stir continuously for 1 minute to coat the onions and cook away the raw flour taste.
Make the creamy sauce.
Gradually whisk in the milk, adding a little at a time to prevent lumps. Stir in the heavy cream. Bring to a gentle simmer and cook for 3–5 minutes, stirring frequently, until lightly thickened. Keep the sauce pourable because it will thicken further in the oven.
Add the seasonings.
Remove the saucepan from the heat. Stir in the Parmesan, 1 tablespoon parsley, thyme, salt, and black pepper. Taste and adjust the seasoning.
Assemble the bake.
Arrange half the sliced potatoes in the prepared dish, overlapping them loosely. Spoon half the sauce over them. Add the remaining potatoes and pour the remaining sauce evenly over the top. Gently nudge the potatoes with a spoon so some sauce flows between the slices.
Bake covered.
Cover the dish tightly with foil and bake for 45 minutes. Carefully uncover and test a potato in the center with a small knife. If it is still quite firm, cover again and bake for another 10–15 minutes.
Finish uncovered.
Once the potatoes are nearly tender, remove the foil and sprinkle with mozzarella if using. Bake uncovered for 15–20 minutes, until the center is completely tender and the sauce bubbles around the edges. The top should have lightly golden patches.
Rest and serve.
Let the bake stand for 10 minutes so the sauce settles and thickens slightly. Sprinkle with the remaining parsley and serve warm.
Notes
Yukon Gold potatoes hold their shape while becoming creamy inside. Russet potatoes also work, but they are more likely to break apart.
The pictured potatoes are fairly thick. If you slice yours thicker than ¼ inch, allow additional covered baking time.
Mozzarella is optional. Leave it off for a creamy herb finish with less of a cheese crust.
Use freshly grated Parmesan when possible for smoother melting.
Tips
Slice the potatoes evenly so they cook at the same rate.
Keep the dish tightly covered during the first part of baking to trap steam and soften the potatoes.
Test the center of the dish before serving; bubbling sauce alone does not mean the potatoes are done.
If the sauce becomes too thick before assembling, stir in a few tablespoons of milk.
Let the finished dish rest rather than serving it directly from the oven.
Avoid broiling a glass baking dish unless its manufacturer explicitly says it is broiler-safe.
Storage
Refrigerate leftovers in an airtight container within 2 hours. Use within 3–4 days.
Reheat individual portions in the microwave, or cover the baking dish and warm in a 350°F (175°C) oven until thoroughly hot. Add a small splash of milk if the sauce has thickened considerably.
Freezing is possible for up to 2 months, although the potatoes may soften and the cream sauce may separate slightly after thawing.