Prepare the mushrooms by brushing the caps clean.
Do not wash them under water.
Season generously on all sides with kosher salt and freshly ground black pepper.
Heat the olive oil in a large skillet over high heat until very hot but not smoking.
Place the mushrooms top-side down and cook until tender and cooked through, about 4 minutes. You'll know they're ready when small pools of juice appear where the stems were.
Gently warm the pitas.
Cut an opening in each pita pocket and slather the inside with mayonnaise.
To assemble, spoon a layer of cooked buckwheat groats into the bottom of each pita.
Add a quarter of the green beans, then stuff one whole cooked mushroom cap inside.
Repeat with the remaining pitas and serve immediately.
Add a clove of minced garlic to the skillet for the last minute of cooking the mushrooms to infuse them with extra flavor.
