Monkfish and Clam Bourride

2 large leek s (white parts only), cut crosswise into 1/4-inch-thick slices

1 (1-pound) piece monkfish fillet, cut into 2-inch chunks

3 tablespoons extra-virgin olive oil

Slow-braised tomato es

1 large fennel bulb (sometimes called anise), stalks discarded and bulb halved lengthwise, cored, and thinly sliced lengthwise

1 cup thinly sliced shallot s (4 medium)

4 garlic cloves,

3 thinly sliced and 1 halved crosswise

24 small hard-shelled clam s (2 pounds) such as littlenecks (less than 2 inches wide), scrubbed well

1/2 cup dry white wine

1/2 cup water

1 teaspoon dried hot red pepper flakes

1 teaspoon finely grated fresh lemon zest

8 fresh basil leaves, chopped

2 tablespoons chopped fresh parsley

4 ( 3/4-inch-thick) slices crusty bread (about 4 inches wide), toasted

Accompaniments: aïoli and green olive tapenade

Garnish: 8 whole fresh basil leaves