2tablespoons chopped chives or finely chopped scallion
1teaspoonfresh lemon juice
Freshly grated nutmeg
Combine the dried porcini and boiling water in a small bowl, cover, and let stand until the mushrooms have softened, about 15 minutes. Drain through a coffee filter, reserving the liquid. Rinse the mushrooms under cool water to remove any grit and pat dry with paper towels. Finely chop by hand or in a food processor. Set aside.
Brush the tops of the mushroom caps lightly with a little of the oil and arrange on an ovenproof platter or baking dish, bottoms up. Sprinkle lightly with salt and pepper. Set aside.
Heat the remaining oil in a heavy skillet, preferably non-stick. Add the onion and sauté over low heat, stirring, until golden, about 5 minutes.
Finely chop the reserved white mushroom stems. You should have about 1cup. Add the stems, the chopped shiitakes, the porcini, and the garlic to the skillet and stir to blend. Cover and cook over very low heat until the mushrooms begin to release some of their liquid. Uncover, raise the heat, and stir in the reserved porcini liquid. Cook over high heat, stirring constantly, until all the liquid has reduced and the mushroom mixture is dry when pressed with the back of a spoon, about 4 minutes.
Heat the oven to 375°F.
Stir the parsley, chives, lemon juice, ¼teaspoon salt, a grinding of pepper, and a grating of nutmeg into the mushroom mixture. Carefully spoon the mixture into the mushroom caps, dividing the mixture evenly. (The mushrooms can be filled several hours in advance and refrigerated, then baked just before serving.) Bake until the caps are tender, 15 to 20 minutes. Serve hot.