Pat lamb dry and season with salt and pepper. In a large skillet, heat oil over medium-high heat. Add lamb and cook until lightly browned, turning with a wooden spoon, about 10 minutes; transfer to a slow cooker. Pour off and discard all but 1 Tbsp. fat from skillet.
Add eggplant, onion, bell pepper and garlic to skillet and cook over medium heat, stirring often, until mixture is slightly softened, about 5 minutes. Stir in tomatoes.
Pour contents of skillet over lamb in slow cooker. Cover and cook on low heat for 6 hours. Serve warm.