1. Preheat oven to 450 degree F and arrange rack in center. Lightly oil a baking pan.
2. Bring 6 cups water to a boil in a large saucepan. Add lentils and simmer until tender, about 15 minutes.
3. Meanwhile, stir together yogurt, cucumber, 1-1/2 teaspoons of the juice, and 1/2 teaspoon of the garlic; salt and pepper to taste, for yogurt sauce. Keep chilled until ready to use.
4. Drain lentils in a colander; transfer to a bowl. Immediately stir in oil, artichokes, spinach, feta, remaining 1 tablespoon plus 1 teaspoon juice and 1/2 teaspoon garlic; salt and pepper to taste. Keep warm, covered.
5. Rinse fillets and pat dry; season with salt and pepper. Dividing pan into 4 sections, arrange 2 lemon slices, side by side, in center of each section. Place 1 fillet on each pair of slices.
6. Roast until fillets are opaque and just cooked through, about 7 minutes.To serve::
7. Divide lentils between 4 serving plates. Place fillets with lemon slices over lentils and drizzle with yogurt sauce. Serve remaining sauce on the side. Makes 4 servings.