Cast on 9 stitches in MC and distribute them evenly over three size 5 DPNs. Join in the round and knit 8 rounds, then work as follows:
1. Switch to CC and Kfb around. (18 st)
2. Purl a row.
3. Switch to MC and Kfb around. (36 st)
Knit 3 rows, then work as follows:
4. (K2, K2tog), repeat to end. (27 st)
5. Knit around.
6. (K, K2tog), repeat to end. (18 st)
7. Knit around.
8. (Slip stitch, K2tog, psso), repeat to end. (6 st)
9. Break yarn, pass through remaining stitches, pull tight. Hide yarn end inside of mushroom cap.
For the cast-on yarn end, turn the tip of the mushroom stalk inside out, work the end in a few rows, then stuff the rest into the cap.
Take a small amount of stuffing and carefully insert it into the cap of the mushroom via the stem. Adjust until the result looks appropriately mushroom-y, something along these lines: