Serves two very generously
Preheat oven to 400 degrees

2 large baking potatoes peeled

Prepare a shallow baking dish
either an 8×8 cake pan, a 9 inch pie plate or a baking dish of comparable size

grease pan lightly
Slice potatoes approximately and no thicker than 1/4 inch thick.

Spread in shingle style layers
after the first layer sprinkle 1 TBLSP flour over potatoes
add next layer of potatoes

1/2 tsp. salt and pepper to taste

slice half of one medium onion
and spread rings over potatoes

1/2 cup celery greens (found in the center of head of celery)

1 TBLSP. butter cut into small pieces and placed on top of other ingredients

3/4 cup of milk preheated
Pour over potatoes

top with 1/2 cup grated cheddar or gruyere

Bake for one and a half hours or until fork can be inserted easily.
If I am short on time and only have an hour for baking I cook this dish in the microwave for 10 minutes before putting it into the oven. It works great to speed up cooking time.
Do NOT skip oven baking though. These potatoes will not be anywhere near as good as they should be if you bake them only in the microwave.

The SECRET of good scalloped potatoes
or
gratin
is to make sure they are well cooked. They should be over cooked rather than undercooked.