Prepare the beef block and bring it to room temperature. Rub salt, pepper, and garlic paste onto the surface.

Heat salad oil in a pan and sear the beef on all sides over medium-high heat until browned. Remove from the pan.

Wrap the seared beef tightly in double-layered plastic wrap and place it on a microwave-safe plate. Microwave at 600W for 1 minute 30 seconds on one side, then flip and microwave for another 1 minute 30 seconds.

Let the beef cool slightly, then refrigerate it wrapped in plastic for resting. Slice thinly to make roast beef.

Cut the sprouts in half and grate the onion. Microwave the grated onion in a microwave-safe container covered with plastic wrap at 600W for 1 minute. Mix the onion with the sauce ingredients (soy sauce, vinegar, sugar, ginger paste, gochujang).