I make my own mushroom spawn by sterilizing rye grain in a pressure cooker. I grow oyster, king oyster, nameko, and reishi.
Portobello is one of the tougher mushrooms to grow. It requires a technique called casing. You can buy a kit online and some people have success, but they are so cheap at the grocery store it is not worth the expense and effort.
So......I'm confused. How do you make your spawn from rye grain?