In a recent New York Times article about boiling pasta, food science writer Harold McGee made an interesting reference to the culinary usefulness of starchy pasta water for both thinning and binding sauces. This is stuff that one can find only in restaurants where a large pot with an inset strainer is used repeatedly to boil many batches of noodles. At home we just don't make enough pasta to have this, and one would have to have a lot of refrigerator space to justify saving pasta water.
But this did get me thinking about things produced just by cooking at home all the time that are worth saving, and can make it very easy to whip something up later without much additional effort. If you look at classic French and European-derived cookbooks, everything comes from stock. There are basic or "leading" sauces made from stock, and then there are hundreds of sauces that can be made by combining, reducing, and augmenting those basic stocks and sauces in different ways. If you stop to think about it, a lot of these ingredients are just byproducts of other processes that are kicking around a busy kitchen where many dishes are made and are available to the chef's creativity.
For instance Charles Ranhofer, the great nineteenth-century chef at Delmonico's, recommended in his monumental The Epicurean, adding more water to the leftover, seemingly cooked out dregs of stock to make another, weaker stock called remouillage (literally "remoistening"), which could be added to dishes that don't call for full strength stock, but could benefit from something richer than water. If one had large stockpots with spigots (so it would be easy to add more water after draining the stock) and not-quite-modern refrigeration as they did in Delmonico's in the nineteenth century, it seems like a very sensible thing to do, particularly in a restaurant where they've got stockpots going all the time, but at home there isn't enough room in the fridge for weak stock. You could add the remouillage back to the original stock though and further reduce it, which is what Thomas Keller suggests in The French Laundry Cookbook as a way to get the most out of those bones.
So what's kicking around in my freezer at the moment? I've got chicken, beef, and veal stock, and demi glace, which is a highly concentrated sauce that can add a layer of rich complex flavor to many dishes. Demi glace is traditionally made by reducing equal parts of veal stock and Espagnole sauce, which is a brown sauce with tomatoes. You don't see Espagnole too much in modern cooking, I think because it's not unlike what you would get if you combined a can of Campbell's tomato soup with an equal amount of beef broth, but if you ever wondered what inspired people to cook with canned tomato soup sometime back in the 1940s, it was probably Espagnole. If you made your own Espagnole with your own beef stock, not only would it be a lot more flavorful than canned soup, but you could serve your family a pâté de boeuf haché à l'espagnole instead of meatloaf with canned soup. I mainly make Espagnole for the purpose of making demi glace, but as long as I've made it, it's not an uninteresting sauce in its own right. I added some raspberries to it recently and served it over chicken breasts that I had in the freezer, and since one usually makes more sauce than one needs for a family of two and a half, I've got some raspberry sauce in the freezer waiting for the right piece of meat or duck to come along.
Then last week I made a batch of beef bourguignon, and at the end of it, I had this concentrated brown wine sauce that was too good to throw away, so I strained it through a fine conical sieve called a chinois to produce a velvety brown sauce that could go well on a steak, and then I had this meat left over in the strainer and thought that that had a lot of interesting concentrated flavor in it that could be added to something else, or maybe I could combine it with a few chopped peeled tomatoes and adjust the seasoning for a quick Bolognese-type sauce.
I generally freeze stock flat in one quart Ziploc bags so the stock can easily be broken up and added to dishes as needed or defrosted in quantity for soups. The concentrated sauces in the photo above are frozen in small four-ounce Rubbermaid containers, which are just the right size for most of the cooking I do at home. Some people like to freeze stocks and sauces in ice cube trays and then bag them up, but I like the flexibility of small individual containers for storage.