Slice pork into matchstick strips. Slice scallions into 1 inch pieces, splitting the white part. Add scallions to pork, add 1/4 tsp. salt, sesame oil, and pepper to pork. Let marinate for 30 minutes.
Shred the cabbage–you want about six cups.
Peel the ginger and garlic, and mince them together. Cut water chestnuts into pieces. Combine corn starch and water, and add it all to the pork mixture.
Add 2 tbs. oil to cast-iron skillet and heat on high until hot. Stirfry cabbage for 45 seconds or until it begins to glisten. Add rest of salt (one teaspoon) and stir-fry for 1 minute more. Remove cabbage from pan, it should be cooked but not limp.
Wipe pan out with paper towel, then add more oil and heat on high until hot. Add pork mixture, and stir-fry until browned. Add garlic and ginger mixture, and stirfry until pork is fully cooked. Return cabbage to pan and stirfry a few seconds longer. Remove from heat and serve.