For the Rice Balls, cook sushi rice as directed on package. Fluff with fork then stir in rice vinegar. Set aside to cool slightly while preparing filling.
To make the shiitake filling, cook bacon in large skillet on medium heat about 7 minutes or until crisp, stirring occasionally. Remove with slotted spoon and drain on paper towel-lined plate. Reserve bacon fat in skillet. Add squash to skillet; cook about 6 minutes or until slightly tender, stirring occasionally. Add mushrooms and green onions; cook about 2 minutes or until softened, stirring occasionally. Add Furikake Seasoning, white pepper and mustard; cook 5 minutes longer or until vegetables are tender. Stir in bacon. Set aside to cool slightly.
For the Ginger Sauce, combine all ingredients in medium bowl. Set aside until ready to serve. Store in airtight container in refrigerator up to 1 week.
To form the rice balls, scoop about 1/3 cup warm rice into palm of hand and make a deep indentation in rice. Place about 2 tablespoons of the shiitake filling in indentation. Using hands, gently shape rice around filling, forming a ball, to enclose filling. Repeat with remaining rice and shiitake filling. Sprinkle rice balls with additional Furikake Seasoning. Serve with Ginger Sauce.
Keep a bowl of cold water nearby when preparing rice balls. If your hands get too sticky while forming the rice balls, wet them in the cold water and continue forming the rice balls.