An ounce of dried porcini in a bowl of hot water for twenty minutes produces two things: mushrooms you’ll use in whatever you’re cooking, and a dark, deeply savory liquid that is the fastest umami stock you’ll ever make. Strain it through a coffee filter to get the grit out. What’s left goes into the braise, into the risotto, into the pasta sauce. It makes everything taste like it cooked for twice as long as it did.
This is what dehydration does to a porcini. It collapses all the water and concentrates every flavor compound until one ounce of dried mushroom does what a pound of fresh cannot. Fresh porcini are beautiful. Dried porcini are useful in a way that’s almost unfair.
Save the soaking water. Every time.