1) Preheat the oven to the temperature of 400°.
2) Remove the skin of potatoes and slice them very thinly (use a mandolin for best results).
3) to prevent discioloration of potatoes, put them into a bowl of cold water.
4) Peel the onions and cut them into thin slices..
5) Drain the potatoes and dry with paper towels.
6) Butter an ovenproof dish
7) Cover the bottom of the dish with a layer of potato.
8) Pat with butter, season and cover with a layer of onion.
9) Again pat the onions layer with butter, season and repeat the layers till all ingredients have been used, finishing with potato layer on the top.
10) Dot the top with the remaining butter.
11) Bake for about 40 minutes till the potatoes are tender and golden.
12) Serve the baked onions and potatoe immediately with green salad.