4medium leeks, white and light-green parts halved lengthwise, washed, and sliced thin (about 4 cups), dark green parts halved, washed, and cut into 2-inch pieces
2cups low-sodium chicken broth
2cups water
4tablespoons (½ stick) unsalted butter
1medium onion, chopped medium (about 1 cup)
Table salt
1small russet potato (about 6 ounces), peeled, halved lengthwise, and cut into ¼-inch slices
1 bay leaf
1(4-inch) sprig fresh thyme or tarragon
1large slice high-quality sandwich bread slice, lightly toasted and torn into ½-inch pieces (see note)
Ground black pepper