Chef John's Irish Stew

Name: Chef John's Irish Stew
Prep time: 20 min
Cook time: 2 h 25 min
Total time: 2 h 45 min
Cuisine: British
Ingredients: 3 pounds lamb shoulder chops; salt and ground black pepper to taste; 1 tablespoon vegetable oil; 1 onion, chopped; 1 tablespoon butter; 2 tablespoons all-purpose flour; 3 cups chicken stock; 0.5 teaspoon dried rosemary; 2 carrots, chopped; 2 stalks celery, chopped; water as needed; 1.5 pounds baby Dutch yellow potatoes; 0.25 cup chopped green onions