Potato Gratin with Yams

I have made this dish few times now and feel that it is time to share. In the past I have only done it with regular  potatoes but then I decided to mix it up a little. I like the look and the taste.

3 garlic cloves
2 tbsp butter
2 large baking potatoes
1 yam same size as regular potatoes
8 oz milk
1 egg
1 cup shredded cheese (Mexican blend or mozzarella - your choice)
1 red onion (optional)

Slice on the mandolin or by hand the yam and potatoes and onion if you wish to use it. spread butter on the bottom of the baking pan and add crushed garlic to the bottom. Layer the potatoes and yam alternately like the roof tiles. Even though the onion is not present in this picture, I have made it with red onion slices in the past and it makes a beautiful and delicious addition to the dish, just place the slices through out the dish. Mix milk and egg and pour it over the potatoes. Salt and pepper and place in the oven that has warmed up to 425 F. When the potatoes are fork tender, about 30-40 min, add the shredded cheese over them. If you like it cheesier add more, it does not hurt the dish at all. when the cheese is melted, in about 5 min the gratin is done.