Clemenza teaching Michael how to make gravy.
Surprised nobody has already mentioned this.
Clemenza and Tessio were with the Don since the family was formed. They have known Michael since he was a baby. The scene is important because it tells us how they perceive Michael within the family at that time.
Clemenza says "Come here kid,you might have to cook for a bunch of guys." That is how little they regarded Michael. Never saw him as a leader.
This is important because Michael goes from being an insignificant civilian to the Don! Notice how they laughed at him when he came up with his idea to kill Sollozo.