Trim roots and any wilted ends from green onions. The cut onions into 1-inch lengths, keeping green and white parts separate; set aside.
Heat oil in a 5- to 6-quart pan over medium-high heat. Add half the pork; cook, stirring, until well browned (about 10 minutes).
Remove from pan and keep warm. Repeat with remaining pork. Discard all but 1 Tbsp drippings from pan. Add sliced onion to pan and cook, strring occasionally until soft (about 7 minutes).
Return pork to pan and add soy, sake, water, and mirin. Bring to a boil; then reduce heat, cover, and simmer for 25 minutes. Add potatoes, white part of green onions, pepper, and sugar. Return to a boil; then reduce heat, cover, and simmer until pork and potatoes are tender when pierced (about 25 minutes).
Garnish with tops of green onions.
Servings: