Prep the Enoki Mushrooms: Cut off the bottom 1–2 inches of the enoki mushrooms. Rinse with water and separate the cluster into small bundles, about 1/4 inch wide.
Assemble the Rolls: Lay a slice of beef flat on a plate or cutting board. Place a small bundle of enoki mushrooms at one end, then roll the beef around the mushrooms tightly. Repeat with the remaining slices until all mushrooms are wrapped in beef.
Mix the Sauce: In a small bowl, whisk together sugar, corn starch, black pepper, soy sauce, mirin, oyster sauce, and water until smooth. Set aside.
Cook the Beef Rolls: Heat a little oil in a pan over medium-high heat. Lay down beef rolls in a single layer and be careful not to crowd the pan. Sear on each side until it becomes golden brown, about 1 minute.
Soak up excess oil: Using a folded piece of paper towel, carefully soak up any excess oil in the pan.
Cook the Sauce: Add the sauce into the pan and let it cook for 1 minute. Move beef rolls around the pan so it soaks up the sauce.
Finish and Serve: Remove the pan from heat. Sprinkle with sliced green onions and sesame seeds for garnish. Enjoy!