Heat 2 tablespoons of the oil in a large, heavy saucepan over medium heat. Add garlic and curry powder and cook, stirring, until garlic is fragrant. Add broth, bay leaf, and lentils and bring to a boil. Lower heat, cover, and simmer 15 minutes. Add potatoes, cover, and cook until potatoes and lentils are tender but not mushy, about 10 to 15 minutes longer.
While lentils are cooking, place kielbasa in a large nonstick skillet, over medium-high heat. Brown on both sides, set aside, and keep warm. Remove tough stems from arugula (or watercress) and coarsely chop. When stew is cooked, toss with remaining 2 tablespoons of oil, lemon juice, and arugula. Season with salt and freshly ground pepper. Top each serving with kielbasa and feta.