2 6-ounce packages albacore tuna packed in water, drained
3/4 cup mayonnaise
1/2 cup chopped green onions
1/4 cup diced seeded English hothouse cucumber
1/4 cup minced fresh dill
1/4 cup drained capers
2 tablespoons Dijon mustard
2 tablespoons minced fresh tarragon
12 1/3-inch-thick slices olive bread
1 cup arugula
Mix tuna, 1/4 cup mayonnaise, and next 6 ingredients in medium bowl to blend. Season with salt and pepper. Divide tuna salad among 6 bread slices; top with arugula leaves.
Spread remaining mayonnaise on remaining 6 bread slices; place atop arugula.
Cut sandwiches in half; wrap each tightly in plastic wrap.
Can be made 1 day ahead. Refrigerate.