For the sauce, into a large bowl, whisk soy sauce, brown sugar, ginger, gochujang, sesame oil, garlic, pear, sesame seeds, and black pepper. Add shredded mushrooms and mix until evenly coated.
In a large cast-iron skillet over medium-high heat, warm oil. Once hot, add onion. Sauté for 2 to 3 minutes, or until onion is translucent and lightly browned. Add marinated mushrooms and remaining sauce and cook for 7 to 8 minutes, or until sauce is absorbed and mushrooms are lightly browned.
Remove from heat and serve hot, divided into bowls with fresh lettuce, steamed rice, and kimchi. Top with chopped scallions and sesame seeds.
