Slice the bread into thin slices, about ½ inch thick. I prefer cutting straight down for small bite-sized portions or at a diagonal for large slicer for more toppings.
Chop the shrimp into pieces. In a bowl, combine shrimp, crab meat, whites and light green parts of the green onions, salt, black pepper, sugar, kewpie mayonnaise, sriracha chili sauce if using, and 1 cup shredded cheese.
Spread the seafood mixture onto the sliced bread using a large spoon. Transfer the assembled bread slices to a baking sheet lined with parchment paper. Top with a sprinkle of remaining cheese. Bake at 400°F for 12 minutes. Transfer baking sheet to the top rack directly underneath the broiler, if needed. Switch to broil mode (400°F) for 3 minutes to melt the cheese on top and to get a light golden brown crust.
Garnish with the reserved green onions and enjoy.
