Whisk the soy sauce, 1 tablespoon sesame oil, the brown sugar and garlic in a shallow dish. Add the steak and turn to coat. Let marinate at room temperature, 20 minutes.
Brush a large grill pan with vegetable oil and heat over medium-high heat. Remove the steak from the marinade (do not pat dry) and grill 2 to 3 minutes per side for medium rare. Transfer to a cutting board; let rest 5 minutes.
Meanwhile, toss the scallions with 2 teaspoons sesame oil. Grill, turning, until charred, about 4 minutes; transfer to the cutting board and cut into 1-inch pieces. Slice the steak against the grain; toss with any collected juices.
Whisk the mayonnaise and gochujang in a small bowl. Toss the kimchi, pear and the remaining 1 tablespoon sesame oil in another bowl. Lightly char the tortillas over a gas burner, about 20 seconds per side (or microwave as the label directs). Serve the steak and scallions in the tortillas with the greens, spicy mayonnaise and kimchi-pear mixture.
Photograph by Justin Walker
Recipe courtesy of Food Network Magazine