Build a multi-level fire in the grill: Leave 1/4 of the bottom free of coals, bank the coals in the remaining 3/4 of the grill so that they are 3 times as high on 1 side as on the other. When the coals are all ignited and the temperature is hot (Hold hand about 5-inches above grill grid, over the area where coals are deepest, for 2 seconds or less), the grill is ready to cook.
Pat the steaks dry with paper towels, then rub them all over with 1/4 cup of the olive oil and sprinkle generously with salt and pepper. Place the steaks over the hottest part of the fire and cook until well seared on 1 side, 6 to 8 minutes. Then move them to the medium hot part of the grill and cook, turning once, until they are done the way you like them, 10 to 15 minutes more for rare. Check for doneness.
Make mushrooms while grilling steaks: Combine the mushrooms, the remaining 1/4 cup of olive oil, and the teaspoon of pepper in a large bowl. Toss gently until the mushrooms are well coated and all the oil is absorbed.
When steaks are done, transfer them to a serving platter, cover loosely with foil, and let them rest while grilling mushrooms. Put the mushrooms on the medium-hot part of the grill and cook, turning occasionally, until they are golden brown and moist all the way through, about 8 to 10 minutes. Slice the cooked mushrooms and put them back in the bowl along with the butter, parsley, and sherry, season with salt and pepper, to taste, and toss gently to coat.
To serve, cut the meat away from the bone, then cut it into thick slices. Divide them among 4 serving plates and spoon mushrooms over the top.