Wow! Mushroom farming just became a billion times easier.
Step 1: Find (or place) mushroom.
Step 2: Fertilize with bonemeal.
Step 3: Harvest giant mushroom!
So far, I've gotten 10-25 mushrooms from one giant. SOUP'S ON!
(mid harvest... red ones are hollow inside the cap!)
Curious side-effect: I planted the mushroom on sand, and after it grew, the sand had turned to dirt.
Protip: Giant red mushroom also makes for a handy quick shelter.