The vegetables in here are what I use, and are open to substitution: The only ones you really need are the onions (or leeks), and celery. I also think fennel adds a lot here. If you can't find it, add fennel seeds, or even a shot of ouzo or other anise-flavored liqueur. Use any seafood you have available.
Heat the olive oil in a large stockpot and sauté the vegetables over high heat for 5 minutes, stirring often, until the onions are translucent. If you are using fresh mushrooms, add them, too. Add the tomato paste, stir well and cook over medium heat for another 2 or 3 minutes, until it darkens.
Add the shells and bodies and bones and smash them all with a potato masher. If you are using dried mushrooms, add them here. Mix well to coat with the tomato paste and to disperse everything. Cook for another 2 to 3 minutes, stirring a couple times.
Add the white wine, bay leaves, peppercorns and parsley and bring to a boil. Let this cook for a minute or two, then pour in enough cold water to cover everything by about an inch; normally this is about 10 cups. Let this simmer very gently for between 30 and 45 minutes. Add salt to taste.
Turn off the heat. Set up another large pot or bowl and put a fine-meshed strainer over it. Line the strainer with a plain paper towel or cheesecloth and ladle the stock through this set-up into the large pot or bowls. Do not try to get the last dregs of stock, as this will be full of debris. Discard the shells, etc.
Let the stock cool for 15 to 30 minutes, then use or pour into quart jars. The stock will keep for a week in the fridge. It will freeze well for 6 months, but after that it deteriorates rapidly.
This recipe makes about 5 quarts.
Keys to Success
Don't let this boil (you want a very gentle simmer) and don't be tempted to simmer it for more than 1 hour or it will lose flavor and get cloudy.
Really wail on the shells, etc. when they go into the pan. You want the pieces small, which will give you better flavor faster. You can put them in a bag and crush them before they go into the pot, too.
All fish and seafood stocks are best immediately, so do yourself a favor and have something that will use them in mind when you start. I really love crab risotto or shrimp risotto made with fresh stock.