Cook the rice in the rice cooker. Mix and cool for at least 30 minutes at room temperature.
Meanwhile, in a medium bowl, combine garlic, ginger, miso, water and mirin.
Cut the beef into 2-inch strips. Transfer to the miso mixture. Season with black pepper. Mix well.
In a large pan over high heat, warm ½ tablespoon vegetable oil. Add the beef and cook until cook through. Taste and adjust with salt and sugar. Transfer to a medium bowl.
Prepare a small bowl of water (drinkable). Wet both of your hands with the water. Sprinkle a pinch of salt on one palm. Rub both hands together. Take ½ cup of warm rice and slightly pat down onto one hand. Make a dent in the middle. Add about 1 tablespoon of beef. Carefully cover up the beef with the rice around the edges and roll the whole thing into a ball. Use both of your palms to gently pressure the rice ball into a triangle shape. Repeat with the rest of the rice.
When ready to serve, wrap each rice ball with a piece of seaweed and sprinkle with furikake. Serve immediately or at room temperature.