Japanese Hamburger Steak

Ingredients

• 1⁄2 cup soy sauce
• 1⁄4 cup bonito flakes
• 2 tbsp. fresh lemon juice
• 2 tbsp. mirin
• 2 tbsp. rice vinegar
• 1 tbsp. fresh lime juice
• 1 (2-inch) piece kombu
• 1 (2-inch) piece ginger, peeled and grated
• 1 slice soft white sandwich bread, crust removed
• 3 tbsp. ketchup
• 3 tbsp. mayonnaise
• 1 tbsp. Worcestershire sauce
• 1 tsp. yellow mustard
• 1 egg
• 1 small white onion, grated
• Kosher salt
• freshly ground black pepper
• 1 lb. ground beef
• 1⁄2 lb. ground pork
• 1 tbsp. canola oil
• 1⁄4 head green cabbage, shredded
• 1⁄2 medium daikon, peeled and finely grated
• Kaiware (daikon) or other sprouts, for garnish (optional)
• Cooked white rice, for serving (optional)