Prep ingredients: Bring a medium pot of water to a boil. Peel and finely chop ginger and 3 large cloves garlic. Wash and drain mustard greens then trim and discard ends. Cut stems and leaves into 2-inch pieces. In a medium bowl, mix pork and five spice powder. In a small bowl, combine tamari and mirin.
Cook spinach: Heat 1 tablespoon oil in a large skillet over high. Add mustard greens, season with salt, and stir-fry until wilted, about 2 minutes. Transfer to a plate.
Crisp pork: Add 3 tablespoons oil to the same skillet over high heat. Add seasoned pork in one layer and cook, breaking up pieces with a wooden spoon, until crispy and brown, 4–6 minutes. Drain all but 1 tablespoon fat from the pan, leaving pork in the skillet.
Add aromatics: Add ginger and garlic and cook until fragrant, stirring, about 1 minute. Reduce heat to medium-high and stir in tamari-mirin mixture, scraping up any browned bits from bottom of the skillet. Stir in tahini and ¾ cup water. Cook until reduced and just a little sauce remains, about 2 minutes. Remove from heat.
Cook ramen: Add ramen noodles to boiling water and cook until tender but still chewy, 2–3 minutes. Drain, rinse with cool water, drain well again.
Finish and serve: Reheat pork and sauce if necessary. Serve ramen noodles in bowls topped with mustard greens and pork sauce. Mix well to combine all the flavors and coat the noodles in sauce. Enjoy!