Wrap the steak in cling film and place in the freezer for 30 minutes to firm up. Unwrap and slice across the grain into thin slices.
In a medium bowl combine the pear, soy sauce, brown sugar, sesame oil, garlic, three balls of stem ginger, 2 teaspoons of the ginger syrup and sriracha sauce (or gochujang if using). Add in the steak, mix well, and marinate for at least 2 hours (or overnight), stirring occasionally.
Add 1 tablespoon of vegetable oil to a cast iron griddle pan over medium-high heat. Add half of the marinated steak to the grill pan in a single layer and cook, flipping once, until the steak is charred and cooked through – this will take about 2-3 minutes per side. Repeat with the remaining 1 tablespoon vegetable oil and steak.
Thinly slice the remaining 2 balls of stem ginger and spring onions.
Garnish the cooked steak with the ginger slices, onions and sesame seeds, if using.