1. Place cucumbers in a food processor; pulse 15 times or until finely chopped. Place in a large bowl. Add radish to food processor; pulse 15 times or until finely chopped. Add radish to bowl. Add green onions and ginger to food processor; process until minced. Add to bowl.
2. Combine vinegar, sugar, and soy sauce; stir until sugar dissolves. Pour over cucumber mixture; let stand at least 15 minutes. Drain in a fine mesh sieve, pressing to squeeze out excess moisture.
3. Preheat broiler.
4. Hollow out top and bottom halves of bread, leaving a 1/2-inch-thick shell; reserve torn bread for another use. Place bread on a baking sheet, cut sides up; broil 3 minutes or until toasted. Layer spinach, salmon, and relish on bottom half of bread; cover with top half of bread. Cut into 4 equal pieces.