How to Make It
In a medium saucepan, heat 1/4 cup of the olive oil. Add 3 of the garlic cloves and the chile and cook over moderately high heat, stirring frequently, until beginning to brown, 1 minute. Add the thyme, oregano, lemon zest strips and bay leaf and cook for 30 seconds.
Stir in the mussels and 1/3 cup of water and season with salt and pepper. Cover and cook, shaking the pot occasionally, until the mussels open, 3 to 5 minutes. Discard any that do not open. Drain the mussels in a colander set over a heatproof bowl. Remove the mussels from the shells and add to the bowl with the reserved cooking liquid. Wipe out the saucepan.
Heat 3 tablespoons of the olive oil in the saucepan. Add the leeks and a generous pinch each of salt and pepper and cook over moderately high heat, stirring occasionally, until softened, 8 minutes. Add the leeks to the mussels. Stir in the vinegar, orange blossom water, if using, and tarragon and season with salt and pepper.
Light a grill or preheat a grill pan. Brush the bread with the remaining 1 tablespoon of olive oil and grill over moderately high heat, turning once, until lightly charred, 5 minutes. Rub the grilled bread with the remaining garlic clove and cut each slice in half on the diagonal.
Arrange the grilled bread on a platter and top with the mussel-and-leek salad. Garnish with the watercress, drizzle with olive oil and serve immediately.