Combine brussels sprouts, sweet potato, scallion, and egg in a large bowl; season with salt. Mix with your hands until vegetables are evenly coated. Sprinkle flour over vegetables and toss to combine (mixture shouldn’t be gummy, but a handful should clump together).
Heat 1 Tbsp. oil in a large nonstick skillet over medium-high. Add half of vegetable mixture; press into a thin even layer across bottom of skillet. Cook until pancake begins to set, about 1 minute. Drizzle 1 Tbsp. oil around edge and shake to ensure pancake can slide around; cook until golden brown underneath, about 2 minutes longer. Slide pancake onto a baking sheet or the underside of a flat pot lid, then invert pancake back into skillet. Drizzle 1 Tbsp. oil around edge of pancake and cook until second side is golden brown, about 2 minutes. Repeat with remaining vegetable mixture and 3 Tbsp. oil to make another pancake. Serve topped with Sriracha, mayonnaise, bonito flakes, and furikake.
