This recipe is from Please to the Table, with some adjustments. I use it all the time, because the mushrooms are very good and can be made a couple weeks ahead of time, if need be. I love food that can be made ahead. (And I *heart* my deep-freeze, too, but that's another post).
Okay. Anya calls for 1 1/2 pounds of white button mushrooms. I always use crimini (or baby bellas, which I believe are just a silly marketing-department-invented name for the same thing). They have a deeper mushroom flavor. I'm sure you could do this with all sort of exotic mushrooms, if you have access to them. Because more often than not I can only get mushrooms in 10 oz packages, I just use two packages.
Trim the stem ends to get rid of the tough bit at the very tip, then wash the mushrooms. Usually, the rule is that you have to wipe each mushroom individually with a damp towel, so that the mushrooms don't absorb excess water. But these mushrooms are going into a pot of water anyway, so don't worry about it. Just wash them. Halve any that are huge. Now put them in a pot and squeeze the juice of one fresh lemon all over them, then let them sit for about 5 minutes while you putter around. Here's the water part: Anya calls for 3/4 cup or enough water to cover. I don't know what type of mushroom she was sampling when she decided 3/4 cup would cover 1 1/2 pounds of mushrooms, but it doesn't work. You can't "cover" mushrooms with water anyway, because they float. But I usually use about 2 cups so that every mushroom is at least somewhat immersed.
Anya leaves you on your own when it comes to adding the salt. I would say I use about two teaspoons. And here's my personal recipe addition: I throw in 1/2 cup dried mushrooms, quickly rinsed, at this point.
Bring the mushrooms to a boil, reduce heat and simmer for ten minutes uncovered. Then take out the mushrooms. Because I use at least twice the water Anya does, I let the mushroom water keep simmering for a couple minutes here to concentrate it a bit. When you decide you've had enough of that, measure out 1 cup of the mushroom-water. Don't discard the rest; throw it into chicken stock. Return the water to the pot along with 1 cup of tarragon vinegar (Anya says 3/4 cup of each, but she's being stingy.) Add a teaspoon of sugar, about a dozen peppercorns, and three bay leaves. If you wanted these hot, this would be a good place to add some red pepper flakes. Bring to a boil, cover, reduce heat, and simmer five minutes. Take off the heat and allow to cool. I've forgotten to let it cool a bunch of times; the mushrooms come out just fine anyway.
Now, pull out a nice glass jar and layer your mushrooms alternately with roughly chopped dill (Anya says 12 sprigs, I just use a goodly amount) and about 5 or 6 garlic cloves, sliced. Pour the vinegar mixture over the mushrooms, and then pour in enough olive oil to cover. Refrigerate.
You have to wait at least two days before these are ready to eat, and a week is better. Serve within a month. I like to dig out the dill, which won't look very nice, but leave the garlic cloves, which taste pretty good.