(sorry, this is not the best picture, but the sun goes down before dinner now!)
1.5 pounds rib-eye steak, trimmed of excess fat
1/4 cup soy sauce
1 tablespoon sesame oil
2 tablespoons dark brown sugar
6 garlic cloves, minced
1 tablespoon finely grated peeled fresh ginger
2 medium red onions, halved and cut lengthwise into 1-inch wedges
1 green bell pepper, seeds and ribs removed, sliced into 1/2-inch strips
4 teaspoons vegetable oil
1 head iceberg lettuce, separated into "cups"
Freeze the beef for 20 minutes; transfer to a clean work surface and slice diagonally (across the grain) into 1/8-inch-thick strips. In a small bowl, whisk together the soy sauce, sesame oil, brown sugar, garlic, and ginger. Place the peppers and onions in a small bowl; toss with half the soy marinade. Toss the steak with the remaining marinade; let stand for 15 minutes.
Heat 2 teaspoons of the vegetable oil in a large nonstick skillet over medium-high heat. Add the onions and peppers; cook until softened, about 5 minutes. Transfer to a plate. Wipe skillet clean with a paper towel.
Heat remaining 2 teaspoons vegetable oil over high heat. Cook half the meat, turning often, until browned, about 2 minutes. Transfer to a plate. Cook the remaining meat. Return first batch and any accumulated juices to the pan; add the onion mixture. Cook, tossing, until heated through, about 1 minute. Serve in lettuce cups.