Braising is the act of cooking something slowly, completely submerged in liquid. My favorite thing about braising is that it is hard to do wrong.
You can cook something for just enough time or a really, really long time. You can cook it on really low heat, or on whatever moderate heat you happen to be cooking something else at the time.
The general idea is that you have an ingredient (say, a duck leg or celery root), or a bunch of ingredients (duck, spices, prunes or celery root, herbs, and spices), to which you add a liquid (stock, wine, water), cover tightly, and cook on low heat for a very long time. In meat, braising will ease tendons, render fat, and soften muscles. For root vegetables, braising softens starchy flesh and perfumes crude flavors.
You can braise other things. Tough greens will bend into blankets; fruit will turn to confection.
Perhaps the best thing about braising is its economy of effort. In the act of cooking one item, you end up with two. The resulting braising liquid–whether of duck or celery root–can strike out on its own and find use in other dishes. Cook sweet potatoes in some duck braising liquid, use it for a rich pasta sauce. Take the resulting “stock” of celery root and poach fish, add to a bloodymary, or make into soup. A recipe, follows:
2 ½ pounds trimmed celery root, about 4 pounds whole, untrimmed
3 cups water
1/3 cup white wine
5 sprigs thyme or 1 or 2 sprigs sage
1 bay leaf
1 ½ tablespoons salt
1 tablespoon olive oil
3 cloves garlic, sliced thinly
½ teaspoon coriander seeds, roughly crushed
Peel trimmed celery root and cut into ½-inch thick slices.
Place celery root in large roasting pan, snugly.
Place water, wine, thyme, bay, salt, garlic, and olive oil in pot and bring to a quick boil. Pour into pan, making sure to just cover celeriac. Cover with foil or snug lid.
Place in oven and cook until celery root is quite tender at its core, about 30 to 40 minutes. Cool in braising liquid. Store in braising liquid. Sometimes, braised vegetables are best made one day in advance. Here's a recipe for Braised Celeriac Soup (using the braised celeriac!)