Mix garlic, grated onion, grated ginger, toasted sesame oil, brown sugar together in a mixing bowl. Stir in grated pear, soy sauce and chilli flakes.
Transfer meat to marinade and toss to coat on all sides. Cover and refrigerate 1 to 2 hours.
Season meat with salt and a drizzle of vegetable oil. Toss.
Brush a cast iron pan with a little vegetable oil and place over high heat. When pan is very hot, add beef in a single layer and cook until meat begins to caramelise around the edges and moisture begins to evaporate, 2 to 3 minutes per side. Serve over hot rice and top with sliced spring onion.
You can marinate the beef for up to 12 hours, but I get good results with a 1 to 2 hour marination time.