- ¼ cup (packed) dark brown sugar
- 3 tablespoons minced peeled fresh ginger
- 1 tablespoon balsamic vinegar
- 1 pound asparagus, trimmed, cut into 1½-inch lengths
- 3 tablespoons peanut oil, divided
- 2 red bell peppers, cut into 1-inch-wide strips
- 1½ pounds crimini mushrooms, halved, each half cut crosswise into ¼-inch-thick slices
- 4 green onions, cut into 1-inch lengths
- Kosher salt and freshly ground black pepper
Combine garlic, sake, soy sauce, brown sugar, ginger, and vinegar in a 13x9x2" glass baking dish. Add meat. Cover and refrigerate at least 2 hours and up to 1 day. Let stand at room temperature 1 hour.
Cook asparagus in a large pot of boiling salted water until crisp-tender, about 4 minutes. Drain. Rinse under cold water to cool. Drain again.
Heat 2 Tbsp. peanut oil in a heavy large skillet over medium-high. Add red bell pepper strips and sauté 3 minutes. Add crimini mushrooms and sauté until mushrooms are soft, about 5 minutes. Add green onions and asparagus and sauté until vegetables are tender, about 2 minutes more. Transfer vegetables to a serving platter. Tent with foil to keep warm.
Heat remaining 1 Tbsp. peanut oil in same skillet over high. Remove steak from marinade; reserve marinade. Add steak to skillet and cook to desired doneness, about 4 minutes per side for medium-rare. Transfer to a cutting board. Tent with foil and let stand 5 minutes.
Meanwhile, place reserved marinade in a small saucepan. Whisk in cornstarch. Whisk over high heat until sauce thickens and boils, about 3 minutes. Remove sauce from heat. Season sauce to taste with salt and pepper.
Cut steak across grain on a diagonal into ½"-thick slices. Arrange steak slices atop vegetables on platter. Spoon some sauce over meat and vegetables. Serve, passing remaining sauce separately.