It goes for $4.99 in my area but I received this courtesy of Jack in the Box.
I've had pretty good experiences with the Buttery Jack line so far and the Portabello Mushroom version was no exception.
The onions were caramelized for a nice sweetness but retained a good amount of watery crispness.
The beef was thick and moist with an enjoyable seasoned flavor that was further complemented by a lot of melted butter... maybe too much butter (Le gasp! I know. I can't believe I said that either). My bottom bun was actually soaked with it. So yeah, it was very buttery.