1/4 cup (1/2 stick) unsalted butter, room temperature
2 tablespoons (packed) minced shallot
1 tablespoon chopped fresh Italian parsley
1 tablespoon chopped fresh chives
1 tablespoon fresh lemon juice
2 garlic cloves, minced
2 teaspoons chopped fresh thyme
1 teaspoon grated lemon peel
4 5x3x1/2-inch slices crusty sourdough bread
1/4 cup extra-virgin olive oil
2 pounds mussels, scrubbed, debearded
Additional chopped fresh chives
Mix first 8 ingredients in small bowl. Season herb butter with salt and pepper. (Can be prepared 1 day ahead. Cover and chill.) Position 1 rack in top third and 1 rack in bottom third of oven; preheat to 500. Brush bread on both sides with oil; place on baking sheet. Place bread on top oven rack; toast until golden, turning once, about 5 minutes per side. Meanwhile, place mussels in wide ovenproof pot.
Cover, place on bottom oven rack, and roast just until mussels open, about 5 minutes. Remove pot from oven and discard any mussels that did not open. Add herb butter to mussels; stir until melted.
Season to taste with ground black pepper. Divide mussels and juices from pot among 4 bowls. Sprinkle with additional chopped chives. Top each serving with sourdough toast and serve.