Parsley is an understated herb. It is in the same family as carrots and celery, which may be why it goes together with them into the stock pot to form the basis of so many soups. These are flavors that you notice most when they are missing. When a soup or a sauce tastes "flat," I usually start by adding a handful of parsley. It is a traditional part of a bouquet garni (a little bag or tied bundle of herbs that go into a soup pot or casserole dish, to be removed later).
In French cooking, a paste made of parsley chopped together with garlic (and sometimes other herbs, along with oil or vinegar) is known as a persillade. This is a pungent ingredient, which can be mellowed by giving it a long cooking time. A similar preparation is gremolata (chopped lemon zest, garlic and parsley, served raw as an accompaniment to the Italian dish ossobuco alla Milanese).
I like to add parsley as a topper for garlic bread, along with a little cracked pepper and fresh grated parmesan cheese. It also goes well in dumplings - just add a double handful to the dough when you mix in the wet ingredients.
To make parsley-cheese biscuits, put two cups of flour in the work bowl of a food processor, then add 4 teaspoons of baking powder and ½ teaspoon of salt. Add 6 tablespoons of butter and a half cup of cheddar, and pulse until the mixture looks like coarse meal. Add ¾ cup of buttermilk and ¼ cup of chopped parsley and continue pulsing until the dough comes together. Turn it out on a board, let it rest for a couple of minutes, then knead it once or twice and pat the dough to less than an inch thick. Cut into circles (use the top of a glass if you don't have a biscuit cutter) and bake for 9-10 minutes.
Parsley really shines in taboulli, a cold grain salad from the Middle East. To make it, soak a half cup of bulgar wheat for a couple of hours. Transfer the bulgar to a paper towel, and squeeze out excess moisture. Chop together 2 bunches of de-stemmed parsley, a couple sprigs of de-stemmed mint, an onion,and half a dozen tomatoes. Combined this mixture with the wheat in a bowl. Make a dressing from a equal amounts of olive oil and lemon juice, and season to taste with salt and pepper. Pour the dressing over the salad and chill for at least a couple of hours.
Parsley also comes to the fore in salsa verde, where it is combined with tomatillos, onions, limes and jalapeno peppers. This mixture gains a depth of flavor with the vegetables (sans parsley) are first roasted until they develop black spots under a broiler or in the oven.
In several areas of the world, cooks fry parsley sprigs. You are likely to find this delicacy in Belgum or Switzerland as a side dish for your fondue, or in Japan wrapped in tempura batter. It is easy to make yourself, if you are already frying something else. Use leftover batter (or pull some out, if you are battering meat, to prevent cross-contamination) and fry for about one minute. Batter tends to stick better if you use curly-leaf parsley (but for most other dishes, try to use flat-leaf parsley - it tends to have more flavor).
Don't forget to dry some parsley. You can even dry the stems of parsley you've used for other dishes, and powder it afterwards in a spice grinder. Dried parsley takes on a sweeter flavor than fresh, and you may find that you actually prefer it for some dishes.
Parsley is such a versatile herb, that from egg dishes at breakfast to jellies at dinner, it can add subtle depth at your table all day long. Experiment a little, and you will develop your own favorite ways to use this important, underappreciated, plant.