Boil the rice with 1 Tbsp. butter and a pinch of salt, covered, on low heat for 55 minutes. Start preparing the remainder about 15 minutes before the rice is ready.
Split one whole onion down the center from the top, and continue slicing half of it lengthwise so that it separates into strips instead of rings (save the other half for something else.) Crush and chop the garlic. Use a cured wok or large heavy frying pan, with a small amount of oil on medium heat. Sautée the Laetiporus for 5-10 minutes, until almost tender. Add the onions and garlic, and turn up the heat to full flame to sear evrything quickly. Add the butter, sherry, rosemary, salt and pepper, turning the heat down just a bit, and cook down for a moment until the liquid is gone. Sear a bit more and remove from heat just before anything burns. Toss the capers in after removing from heat. Pile the rice onto a serving platter, pile the sautéed mushrooms on top, and serve.