Preheat oven to 400 degrees F and butter a 9×13 glass baking dish.

In a medium-size pot, melt butter over medium heat. Mix in the flour, garlic powder, and salt. Stir constantly for one minute. 

Stir in milk. Cook until mixture has thickened.

Stir in cheese all at once, and continue stirring until melted, about 30 to 60 seconds.

Layer 1/2 of the potatoes into bottom of the prepared casserole dish and top with the onion slices. Then pour half the cheese mixture on top.

Repeat step 5 with remaining potatoes, onions, and cheese mixture.

Cover with aluminum foil and place in oven for 1 hour. Remove foil and bake for an additional 30 minutes. (Or longer if you desire a crisper finish.)

Set aside to rest for 15 min then it's ready to serve.