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.
I didn't really notice the peppery quality of the mayo over all the butter.
Overall, Jack in the Box's Portabello Mushroom Buttery Jack was really good all around if a bit overly rich due to the butter (not really complaining). Both the mushrooms and Swiss cheese are front and center. Compared to Carl's Jr.'s recently-introduced Mushroom & Swiss All-Natural Burger, I have to tip my hat to Jack's version, which is smaller but comes with a much thicker, meatier patty and better quality mushrooms and Swiss.
Nutritional Info - Jack in the Box Portobello Mushroom Buttery Jack (275g)
Calories - 807 (Calories from Fat - 462)
Fat - 51g (Saturated Fat - 23g)
Sodium - 1081mg
Carbs - 51g (Sugar - 12g)
Protein - 38g