In a small mixing bowl, stir together soy sauce, hoisin sauce, rice vinegar, honey and water until smooth. Set aside.
In a large skillet over low heat, add sesame oil, garlic and chili flakes. Cook stirring occasionally until fragrant, about 5 minutes. Remove from heat and very carefully pour chili oil into a heat-proof bowl, leaving about 2 tablespoons in the skillet. Set aside remaining chili oil.
Increase heat to medium and add ground pork or chicken to the skillet. Season with pepper and cook until browned and cooked through, breaking up with a wooden spoon, about 5 minutes. Add mushrooms and chopped white scallion and cook 2 minutes longer. Slowly add 1/2 of the soy sauce mixture and simmer until sauce thickens and the meat and vegetables are caramelized and crispy, about 5 minutes. Remove skillet from heat. Stir in about 3 tablespoons chili oil and cover to keep warm.
Meanwhile, cook noodles according to package directions. Drain and set aside. In the pot used to cook the noodles, simmer broth and remaining 1/2 of the soy sauce mixture over medium heat. Return noodles to the pot, add spinach and toss to combine. Serve noodles topped with some of the meat, scallions and additional chili oil, if desired.