6 tablespoons unsalted butter3/4 cup chopped leeks, white parts only3 medium carrots, peeled and cut into 1/2-inch dice3 medium parsnips, peeled and cut into 1/2-inch dice1 large fennel bulb, stalk cut off and bulb halved, cored and chopped3/4 teaspoon salt, plus more to taste3 1/2 tablespoons flour1 1/2 cups half-and-half1 1/2 cups chicken or...