Tangy Tuna Salad Sandwich

Ingredients

• 1 eight-inch, whole-wheat baguette or 8 ounces whole-wheat boule
• 1/2 red onion, about 1 cup, thinly sliced
• 1/2 cup reduced-sodium feta cheese, crumbled
• 1/2 English cucumber, about 1 1/2 cups, chopped
• 3 Roma tomatoes, sliced
• 1/4 teaspoon black pepper
• 1 head red leaf lettuce, chopped
• 1/2 cup reduced-sodium tuna, packed in water, drained
• 1 tablespoon white wine vinegar
• 1 teaspoon Dijon mustard
• 1 tablespoon olive oil