Budget Scalloped Potatoes

Scalloped potatoes are an inexpensive demonstration of two useful skills: cutting food to an even thickness and thickening milk with a flour roux. The potatoes cook at the same rate, while the sauce sets around them instead of pooling at the bottom.

Needed equipment

• 9-by-13-inch baking dish or similar 3-quart casserole
• Saucepan
• Sturdy spoon or fork
• Knife and cutting surface
• Foil, an oven-safe lid, or an inverted sheet pan large enough to cover the dish

Optional equipment

• Mandoline used with its guard for faster, more even potato slices
• Whisk for smoothing the sauce more quickly

Ingredients

• 2 1/2 pounds russet or Yukon Gold potatoes
• 1 medium onion, thinly sliced
• 3 tablespoons butter or neutral cooking oil, plus more for the dish
• 3 tablespoons all-purpose flour
• 3 cups milk, warmed if convenient
• 1 1/4 teaspoons salt, plus more to taste
• 1/2 teaspoon black pepper
• 1/2 teaspoon garlic powder, optional
• 1/2 teaspoon dried thyme, optional
• 1 cup shredded sharp cheddar, optional

Instructions

1. Heat the oven to 375°F. Lightly grease the baking dish.
2. Scrub or peel the potatoes. Slice them evenly about 1/8 inch thick. If using a mandoline, set it up on a stable board, use the food holder or a cut-resistant glove from the first slice, and stop with a safe piece remaining. Do not freehand the final piece.
3. Melt the butter in the saucepan over medium heat. Add the onion and a pinch of salt. Cook until softened but not deeply browned, 5 to 7 minutes.
4. Sprinkle in the flour and cook, stirring, for 1 minute. Slowly add the milk while stirring firmly and scraping the bottom. Add the measured salt, pepper, garlic powder, and thyme. A whisk makes this faster but is not required.
5. Bring the sauce to a gentle simmer, stirring across the bottom, until it lightly coats a spoon, 3 to 5 minutes. It should remain pourable because the potato starch will thicken it further.
6. Arrange one-third of the potatoes in the dish, overlapping them slightly. Spoon over one-third of the onion sauce. Repeat twice. Press gently so sauce reaches between the slices. Add cheese between layers if using it, reserving a small handful for the top.
7. Cover tightly and bake until a knife slides through the center with only slight resistance, 45 to 55 minutes.
8. Uncover, add reserved cheese if using, and bake until the center is fully tender and the top has browned spots, 20 to 30 minutes more.
9. Rest for 15 minutes before serving. The sauce will finish thickening as bubbling settles.

Sensory indicators

The covered dish should bubble around the edges before it is uncovered. A thin knife should pass through several center slices without crunching. The finished sauce should cling to the potatoes rather than run like milk or sit in rubbery clumps.

Common mistakes

If the top browns while the potatoes are still firm, cover the dish again and continue baking. If the sauce is watery, let the dish rest before changing anything; potatoes release moisture and the sauce thickens as it cools. If it remains thin after resting, the slices may have been very wet or the roux may not have simmered.

If the sauce becomes lumpy in the saucepan, take it off the heat and whisk vigorously. A stick blender can rescue it before layering. Review how flour thickens a sauce before the next batch.

Substitutions

Yukon Gold potatoes hold their shape and taste buttery; russets soften more and release extra starch. Use either or combine them. Unsweetened unflavored nondairy milk can replace dairy milk, but thin varieties produce a looser sauce. Oil works in the roux when butter is unavailable. Cheese makes the dish closer to potatoes au gratin but is not required.

Storage and reheating

Cool promptly and refrigerate covered for up to 4 days. Reheat individual portions covered in the microwave, or reheat the dish covered in a 325°F oven until hot throughout. Add a spoonful of milk around dry edges before reheating.

Variations

Add diced cooked ham, sauteed mushrooms, or chopped spinach between the layers. Keep additions modest and drain wet vegetables well so they do not dilute the sauce. For a crisp top, add buttered breadcrumbs during the final 15 minutes.