Separate the stems of the porcini from the caps and scrape the caps and stems gently with a small knife to remove any clinging dirt, then wipe with a damp cloth. Slice the stems and caps. Cut the guanciale into small pieces. Drain the tuna and break it up with a fork.
Crush the garlic cloves and put in a saucepan with the oil. Sauté over medium heat for a couple of minutes, or un