Wrap steak in plastic wrap, and place in the freezer for 30 minutes. Unwrap and slice across the grain into 1/4-inch thick slices.
In a medium bowl, combine pear, soy sauce, brown sugar, sesame oil, garlic, ginger and gochujang. In a gallon size Ziploc bag, combine soy sauce mixture and steak; marinate for at least 2 hours to overnight, turning the bag occasionally.
Heat 1 tablespoon oil in a cast iron grill pan over medium-high heat.* Working in batches, add steak to the grill pan in a single layer and cook, flipping once, until charred and cooked through, about 2-3 minutes per side. Repeat with remaining oil and steak.
Serve immediately, garnished with green onions and sesame seeds, if desired.