Combine the soy, mirin, garlic chili sauce, ginger, garlic and honey or agave in a non reactive dish or resealable bag. Add the pork and let sit at room temperature for 20 minutes or refrigerate up to 1 hour.
While the pork is marinating, chop the vegetables and set aside. Strain the kimchi reserving the liquid.
Drain the liquid from pork and reserve.
Heat a wok or large heavy skillet over medium-high heat. Add the canola oil and when smoking, add the pork and cook until just crisp, about 3 to 5 minutes, being careful not to overload the wok or pan, or the meat will steam. Remove the pork to plate and keep warm. Add the onions, spinach and kimchi and 1/2 of each reserved liquids (the pork marinade and kimchi liquid). Add more if you like extra sauce. Cook until pork is still tender and spinach is just wilted, about 3 to 4 minutes more. Transfer the pork mixture to a serving dish.
Garnish with cilantro and a drizzle of hot chili oil, if using. Serve with rice.