This was a treat I didn't expect. I had a feeling there was potential, but I really had no idea it would be as good as it was. It wasn't difficult, either. A lot of things to pull together, but most could be done in advance, which made things easy.
I think this would be easier on the grill, simply because the pineapple, buns and 'shrooms could be all done together, with the rest prepped, and ready for final assembly, but in the middle of winter, and extremely cold temperatures, we had to do it inside. It worked just fine.
1/4 cup mayonaise
2 teaspoons teriyaki sauce
Brown butter bread crumbs (adapted from the Smitten Kitchen)
4 tablespoons salted butter
1 small shallot, minced
1 garlic clove, minced
1 cup panko bread crumbs
2 large portobello mushroom caps, stems removed
Fresh cracked black pepper
4 onion, kaiser or sourdough burger buns, split
1 small red onion, thinly sliced
4 fresh pineapple slices (canned works, too)
In a small bowl, combine mayonaise and teriyaki sauce, and stir well. Transfer to an airtight container, and refrigerate until ready to use.
Heat oven to 450 degrees. With a clean, damp cloth, clean mushroom caps, by gently rubbing them.
Generously oil a baking sheet. Place mushrooms on the baking sheet, gills up. Generously drizzle with olive oil, and season with salt/pepper. Transfer mushrooms to the heated oven, and bake 15 minutes. Remove from oven, flip mushrooms, and continue baking another 15 minutes, or until deeply browned with a light char.
In a skillet set over medium heat, melt butter. Stirring constantly, simmer butter until the milk solids begin to brown. Quickly add shallots and garlic, and saute for about a minute. Add panko bread crumbs, and continue cooking until they begin to brown, about a minute. Remove from heat, and set aside. Note: I made mine the night before, and refrigerated them. I microwaved them for about 30 seconds before I wanted to serve them, and it worked just fine.
Generously spread teriyaki mayonaise onto the split sides of the buns. In a skillet set over medium high heat, with mayonaise sides down, brown buns. When golden brown, remove from skillet and set aside. Add pineapple slices to the hot skillet, and sear until a nice char forms on each side of the pineapple. Remove from heat.
When mushrooms are done, slice them about 1/4" thick, and begin layering your sandwiches - one large mushroom is enough for two sandwiches. Layer mushrooms first, then bread crumbs, pineapple, a small dollop of teriyaki mayonaise, and an thin onion slice. Serve immediately.