Guild Wars 2 Portobello Mushroom Farming Locations
Portobello Mushrooms are one of the few different Mushrooms you can find in the game. As they are in real life they're more expensive than just the regular mushroom and a lot more rare than the regular type of mushroom. While not impossible to obtain you'll get them about 1/3 the amount of time until you get into 60 - 70 zones where you'll find Portobello Mushroom nodes all the time.
Portobello Mushroom Farming in Mount Maelstrom
Northern Mount Maelstrom in Gauntlet Gulch by the Gauntlet Waypoint.
This is a great location that you can return to every hour and a half to gather up all of the Portobello Mushrooms again. Sometimes they spawn sooner, I am not 100% sure of their spawn times.
Below is a picture of how many Portobello Mushroom spawn locations are here.