Preheat oven to 375°F

Mince garlic and sautee with butter in sauce pot

Gradually add in flour mixing well

Gradually add in all the milk into the flour/garlic/butter mix

On medium heat, stirring constantly (around 8 minutes), until it comes to a slow boil.

Once mixture becomes thicker and has come to a boil, remove from heat

In a greased casserole pan, layer sliced, potatoes in a layer. Add diced onions layer and chunks of corned beef. Salt and pepper. Pour half of the milk mixture over the layer. Repeat for a second layer

You can add shredded marble cheese at this time if you like.