All these toon shaders I have been seeing render an duplicated mesh behind it and extrude the normals of the vectors to make it look like it has a border, but it feels a bit hacky to me.

Basically I am trying to create an outline shader for a plane so these toon shaders do not really work.

It's probably due to my lack of deep knowledge about shaders in particular but I am really confused and would like someone to explain. I'm also a bit worried that rendering an object twice would hit the performance.