How to attache multiple nodes of a node type to another node of another node type?
Install the references module, once you done that, you can enable the node_reference submodule and attach a "Node Reference" field to your "restaurant" content type.

Next, set the "item" content-type as reference for the field and the "Number of values" setting as "Unlimited".

When creating or editing a restaurant, you should now be able to link multiple menus to the node.