Prep mushrooms & beef: Trim the enoki mushroom, wash, and separate them into small bundles. Lay a strip of thinly sliced beef flat on a clean surface. Place a bundle of enoki mushrooms at one end of the beef, aligning the mushroom tips with the edge of the beef. Then roll it up tightly so the beef wraps snugly around the mushrooms. Repeat and set the finished rolls aside.
Make the sauce: Mix everything in a bowl — gochujang, gochugaru, sugar, soy sauce, garlic, black pepper, and water.
Cook: Heat a pan over medium heat. Add a little oil and place the beef rolls seam-side down. Let them brown a bit on all sides, then pour in the sauce. Cover and simmer for a 2-3 minutes until the beef is cooked through and the sauce thickens slightly.
Garnish & serve: Top with green onions and red pepper slices if using. Serve with hot steamed rice. Or toss some instant noodles in that sauce.