By cooking the mushroom you removed the poison!
The item id for Roastedmushroom 0 in unturned is 2011. You can use this ID with the commands below to spawn the item, give it to a player or buy it on a modded economy server.
Useable: Consumeable
Size_X: 1
Size_Y: 1
Size_Z: 0.2
Food: 20
Energy: 30
Blueprints: 1
Blueprint_0_Type: Supply
Blueprint_0_Supplies: 1
Blueprint_0_Supply_0_ID: 2010
Blueprint_0_Supply_0_Amount: 1
Blueprint_0_Level: 2
Blueprint_0_Skill: Cook
Blueprint_0_Build: 27
Asset_Bundle_Version: 2