In Germany, where I grew up, the Thursday before Easter is known as Gründonnerstag, or Green Thursday. The holiday, which commemorates Christ's Last Supper, dates back to the Middle Ages, when parishioners were given a green branch to signify completion of Lenten duties. Over time the "green" has also come to signal the arrival of spring, with its profusion of fresh herbs and vegetables, which are cooked into such holiday dishes as kerbelsuppe, a delicate chervil soup, and spinatknöpfli, a green variation of spätzle made with spinach and drizzled with brown butter. Though I now live in the U.S., I still observe Gründonnerstag each year, using fresh sorrel from my garden to make a tangy sauce that I serve over baked trout. It's my way of putting both Lenten fish and spring herbs together on one very verdant plate.