Begin by cutting the Spam into small cubes for uniform cooking. Heat a non-stick pan over medium heat, then add the Spam. Cook for about 5-10 minutes, stirring occasionally, until the cubes are crispy and nicely browned.

In a small bowl, mix together low-sodium soy sauce, mirin, and white sugar to create a glaze. Lower the heat under the Spam pan to low, then pour the glaze over the crispy Spam. Simmer for about 3 minutes until the sauce thickens.

Preheat your oven to 350°F (175°C). In a 9x13 casserole dish, spread the cooked sushi rice evenly across the bottom. Sprinkle a generous layer of furikake, followed by the glazed Spam. Drizzle Japanese mayo on top and finish with an additional layer of furikake.

Place the assembled casserole under the broiler at 500°F (260°C) for approximately 3 minutes until the top is golden and slightly crispy.

Once broiled, remove the Spam Musubi Bake from the oven and allow it to cool slightly. Top with freshly sliced green onions and serve warm alongside roasted seaweed for hand rolls.