Savory portobello caps are the “meat” of this vegetarian main dish, their chewy flesh sliced into thin strips like flank steak. For polenta lovers, microwave ovens are a boon, making the cornmeal dish fast, fussless, and foolproof (albeit less luscious than when long-cooked). The recipe yields a soft mush. If you prefer solidity, reduce the liquid by ½ cup and cook the polenta a few minutes longer.
Please note that broilers differ so dramatically that there is no way to provide uniform directions: You must know your own. The goal is to cook through and crisp the mushroom caps—not to sear or steam them.