Place flour in a resealable plastic bag; season with salt and black pepper. Add lamb to the bag; shake until evenly coated with flour.

Melt 3 tablespoons butter in a large pot over medium-high heat. Add lamb, in batches; cook until golden brown on all sides, about 5 minutes per batch. Set aside.

Reduce heat to medium; add remaining 3 tablespoons butter. Stir in onions; cook until softened and translucent, about 5 minutes. Stir in apple, chicken stock, raisins, brown sugar, curry powder, and browned lamb; bring to a boil over medium-high heat. Reduce heat to medium-low, cover, and simmer until lamb is very tender, 1 to 1 1/2 hours. Stir in lemon juice; cook 2 minutes.