This was great....although I made a couple changes. I only had 3 caps, so I took an onion and cut it in 1/2 and broiled it as well with the extra sauce I had. I toasted bread with mozzerella cheese and garlic salt on it. I used one piece of the cheese toast bread on one side and one piece of plain toasted bread for the other side and spread miracle whip on it after toasting. I didn't have any capers and it was still great. I used the broiled onions to spread over the sandwich instead of lettuce and tomato. I thought the combination of grilled onions and melted cheese on toasted bread, made this perfect. Great recipe, thanks so much for posting.

Try replacing the mayo with goat cheese - unbelievable! You can also add a sliced vidalia and a whole red pepper to broil with the portabello - make sure to peel your red pepper after. I use potato rolls. Very addictive.

These were very tasty sandwiches and easy to make! I watched them very closely while broiling to make sure they didn't burn.

Very tasty. I would let the mushrooms sit out a few minutes after cooking before putting them on the buns. It was a bit messy, but VERY tasty.

My husband and I are crazy about portobellos and make this recipe all of the time. It takes great with aioli sauce and melted provolone cheese on top. Would eat it every other night if possible!

I've made these with just the marinade, not in sandwich form with the capers and mayonnaise.

MMMM, these were very tasty and easy to make. I used whole wheat buns, but didn't change anything else. Thanks for sharing!

These sandwiches are delicious, but have plenty of napkins because these bad boys are JUICY!!!

These were good but I did change them so I don't know what the actual recipe tastes like exactly. I didn't have capers so I used garlic mayonnaise instead. I added a little chili pepper to the marinade and marinaded for two hours. I loved them, they were sooo yummy and my husband liked them too and he does not like mushrooms. I will definitely make these again, and would love to try them with the caper mayo.

These were very good. Must have again.

This was really good and easy. I followed the marinade recipe exactly, except I used fresh thyme. I set the burgers up a bit differently, though: I didn't have capers, so I used a balsamic mustard that has a similar tangy flavor; I used sprouted-grain buns; and I used alfalfa sprouts instead of lettuce. Definitely a keeper!

I add 1/4 t of lemon juice to the mayo mixture but otherwise follow the recipe exactly, and they are absolutely incredible. Everybody who I've fed them to devours them. I prefer these over meat any day. I make them all of the time, sometimes on the grill, sometmes in the toaster oven. In my opinion, they go well with a good merlot.

I do not like Mushrooms, so I did not like this very much, but my husband loves Potobello, and he likes this sandwich. He didn't love it because it needed little more spice, was a little bland. Capers defenitely add that much needed "Oomph"!! I saturated portobello with marinate for almost an hour, which was good. They became crispy on the outside and juicy on the inside. Took the hint from other reviewers and put the shelf one down in oven.

Made it exactly as the recipe said. Delicious, though I do agree with other readers when they say it needs to be served on something heartier than a hamburger bun or the bun will get soggy. Thanks for the great recipe!

The write-up for this recipe was right - - it is juicy! My grocery store did not have focaccia bread when I got there after work so I used onion flatbread. The focaccia probably would have soaked up some of the juices. But don't get me wrong, this was very, very good. Very satisfying sandwich!

The flavor was great. I grilled them on a HOT grill pan instead as to avoid the possibility of burning them. They turned out great. I don't like capers so I didn't use them. I use some slices of fontina cheese and rubbed some garlic on my toasted kaiser rolls and it was yummers!