Combine sweet potato, 2 teaspoons oil, and 1/4 teaspoon salt in a shallow roasting pan coated with cooking spray; toss to coat. Spread into a single layer. Bake at 450° for 40 minutes or until tender and edges begin to blacken.
Sprinkle lamb with remaining 1/4 teaspoon salt and curry powder; toss well to coat evenly. Heat 1/2 teaspoon oil in a large Dutch oven over medium-high heat. Add half of lamb; cook 6 to 7 minutes or until browned, stirring occasionally. Remove lamb from pan; keep warm. Repeat procedure with 1/2 teaspoon oil and remaining half of lamb. Add remaining 1 teaspoon oil to pan. Add onion, garlic, and 1/2 cup broth; cook 4 minutes or until onion is tender, scraping pan to loosen browned bits.
Place half of roasted sweet potatoes and remaining broth in a food processor or blender; process until smooth. Add pureed mixture to onion mixture in pan. Stir in remaining half of roasted sweet potatoes, lamb, half-and-half, and ground red pepper, and cook over medium heat 5 minutes or until thoroughly heated.