Slice the pork tenderloin and pork belly into very thin, bite-sized strips.
In a small glass bowl, whisk together the gochujang, tamari, and sesame oil until smooth.
Heat a large wok or stainless steel skillet over medium-high heat.
Add the pork belly strips to the dry pan first, sautéing until the fat renders and the edges become golden and crispy.
Add the pork tenderloin strips and black pepper to the skillet, searing for 3-4 minutes until browned and cooked through.
Stir in the minced garlic and grated ginger, cooking for 30 seconds until the aroma is released.
Toss in the chopped bok choy and sliced shiitake mushrooms, stir-frying for 2 minutes until the vegetables are tender yet vibrant.
Pour the gochujang sauce over the mixture, tossing constantly for 1 minute until the sauce thickens into a glossy glaze.
Transfer to a plate and garnish with thinly sliced green onions and sesame seeds.