Heat 2 tablespoons of the oil in a large, heavy saucepan over medium heat. Add the garlic and curry powder and cook, stirring, until the garlic is fragrant.
Add the broth, bay leaf, and lentils and bring to a boil. Lower heat, cover, and simmer for 15 minutes.
Add the potatoes, cover, and cook until the potatoes and lentils are tender but not mushy, about 10 to 15 minutes.
Meanwhile, place the kielbasa in a large nonstick skillet over medium-high heat. Brown on both sides. Set aside and keep warm.
When the stew is cooked, stir in the remaining 2 tablespoons oil, lemon juice, arugula, and ¼ teaspoon each salt and pepper.
Divide the stew among bowls and top each with the kielbasa and Feta.