In 10-inch skillet over medium heat, in hot oil, cook lamb, a few pieces at a time, until browned on all sides. Transfer to paper towls to drain.
Reduce heat to low. To hot drippings, stir in curry powder, garlic, ginger and cumin. Return lamb to skillet; stir to coat with curry mixture. Stir in soup, water, and tomatoes. Cover; cook 45 minutes, stirring occasionally.
Add potatoes; toss gently to coat. Cover; cook 20 minutes more or until potatoes are tender. Garnish with lemon slices.