Ingredients:
2 lbs large fresh mushrooms (plain or cremini)
1 medium-to-large onion, minced
2 medium-sized leeks, minced
1 large or 2 small ribs celery, minced
1 8-ounce can mushrooms, diced finely
2 heads fresh garlic
1 or 2 eggs, beaten
Olive oil for sauteeing
Salt, pepper, and stock to taste

(Your best bet is to have all these vegetables cut up in advance, but you may not have the time to plan that far ahead. Also useful is an accomplice to help you with the chopping. If you can't arrange for either of these, be sure to do the sauteeing on a low flame, so you won't wind up cooking faster than you have your ingredients ready. Note also that you're going to need a lot of medium-sized bowls to put your veggies in, smaller ones for the chopped and one larger one for the already-sauteed.)

Start with roasting your garlic. (I like to use the following method for roasting; you may have a different one you prefer.)
Separate garlic into cloves, but do not peel. With a sharp knife, cut off the tip of each clove (where it joined to the head) to make it easier to remove the cloves from the peel after roasting. Place garlic in a small oven-safe dish and add a few tablespoons of olive oil (enough to coat all of the cloves, with a little extra). Put dish in oven at 350-375 degrees until cloves are soft and lightly browned. Check periodically while the rest of the stuff is cooking, and take out before it gets too brown.

While garlic is roasting, start your onions sauteeing in a large and deep skillet or wok, stirring occasionally. While they're sauteeing, take out your mushrooms. Remove and save the stems, and clean or peel the caps. Arrange caps concave-side-up in a large roasting pan. Take the stems (and any mushrooms that came apart when you first took off the stems) and mince them finely. Put aside along with the rest of your minced vegetables.

When the onions are starting to turn brown, remove them from the wok into your largest side bowl. Start the leeks sauteeing; once they begin to go soft and translucent, pour the onions back into the skillet. Stir well and continue to cook for a minute or so, then remove all the contents of the skillet into the side bowl. Deglaze the pan with a spoonful or two of stock, then pour the stock into the side bowl and add a bit more olive oil to the pan. Repeat the sautee-combine-remove process for the celery (until translucent and aromatic), the mushroom stems (until the liquid exuded begins to evaporate), and the canned mushrooms (until the liquid is entirely gone and they begin to brown on the edges).

By the end of the process you should have all the vegetables cooking together in the skillet, and the garlic should be done roasting. Let the garlic cool a bit while cooking down the vegetables -- the mixture should smell rich and earthy, and shouldn't have too much liquid -- then squeeze the garlic cloves out of their husks, mash them into a paste, and combine them with the mixture. Remove from heat and allow to cool a bit, while adding salt and pepper.

Add a spoonful of the vegetable mixture to the beaten egg and stir, then pour the egg mixture back into the vegetable mixture and blend thoroughly. (This keeps the heat of the mixture from cooking the egg too soon.)

Using a tablespoon, start scooping the stuffing mixture into the mushroom caps. There should be enough to heap each mushroom cap, possibly with some left over. Spray the top of the stuffed mushrooms with cooking spray, sprinkle with a little parsley, and bake at 350^o for about half an hour or until stuffing mixture is set. If you've got leftover stuffing mixture, you can put it into a small oven-safe dish (greased) and bake it on its own.

Serve hot and watch them disappear!