Melt butter in a saute pan over medium heat. Add onion and saute until soft, about 5 minutes.
Sprinkle flour and bring to a simmer.
Slowly add milk, stirring constantly. Add salt, pepper and thyme. Bring to simmer, stirring constantly.
Remove from heat and stir in cheddar cheese until melted.
Use cooking spray to coat 6”x8” oven-proof baking pan or 6 cup casserole.
Lay down ⅓ of potatoes and cover with ⅓ of the sauce. Continue layering until all potatoes and sauce are used, leaving enough sauce to completely cover the top of the potatoes.
Bake, uncovered, until potatoes are tender, 60-70 minutes.
* Any well-melting, hard cheese can be substituted for cheddar so feel to use whatever you have on hand. Alternately, cheese can be eliminated entirely to make scalloped potatoes. In this case, increase the milk by ½ cup.
Hack: Leftovers can be stored in the refrigerator for up to 5 days or frozen in serving-sized portions for up to 3 months.
Hack: This recipe is happy to share the oven with other baked items such as a roast or meal loaf. Adjust the cooking time of the potatoes to account for any change in oven temperature.
Keyword au gratin potatoes, baked potatoes, baked side dish, casserole side dish, cheesy potatoes, potato casserole, potato side dish, scalloped potatoes, vegetarian, vegetarian side dish