In a large saucepan, heat oil over medium-high heat. Add beef and onions and brown for 5 minutes, turning once.
Add chicken broth and 1/2 cup water and bring to a simmer, scraping up any browned bits on bottom of pan. Add soy sauce, mirin, ginger and black pepper.
Zest lemon and add to stew. Juice the lemon and reserve the juice.
Cover and simmer for 45 minutes, stirring occasionally. Add sweet potato; simmer for an additional 15 minutes. Add celery and peppers and simmer for an additional 15 minutes.
In a small dish, mix cornstarch with 2 tablespoons of water. Stir into the simmering stew and cook for an additional minute.
Stir in lemon juice to taste and serve. Garnish with parsley if desired.