Grilled Fresh Tuna Salad Niçoise

Ingredients

• 1 pound small red-skinned potatoes, cut into eighths
• 1/3 cup fresh lemon juice
• 2 tablespoons olive oil
• 2 tablespoons Dijon mustard
• 1/2 teaspoon salt
• 12 ounces green beans, cut into 2-inch lengths
• 2 tuna steaks (1 inch thick, 1 1/2 pounds total)
• 1/2 teaspoon freshly ground black pepper
• 1 pint cherry tomatoes, halved
• 1 bunch watercress, tough stems trimmed
• 1 endive, cut crosswise into 1/2-inch slices