1/2 cup toasted shelled unsalted pistachios, coarsely chopped
In a small saucepan, boil 2 cups of water. Add the sugar, 1 1/2 teaspoons of salt and 1/2 teaspoon of pepper; stir until the salt dissolves. Add the lemon slices and cook over moderately high heat until softened, 15 minutes. Transfer the lemon slices to a plate. Boil the liquid until reduced to 1/3 cup, 3 minutes; let cool. Whisk in 1/4 cup of the oil.
Meanwhile, bring 3/4 cup of water to a boil in a saucepan. Add 1/2 teaspoon of salt and the couscous. Remove the pan from the heat, cover and let stand for 5 minutes. Fluff the couscous with a fork.
Bring a medium saucepan of water to a boil, add the lentils and cook over moderately high heat until tender but not mushy, 6 minutes. Drain, transfer to a bowl and add the couscous.
Stir all but 1 tablespoon of the lemon oil into the couscous and season. Toss the arugula with the lemon juice and remaining olive oil; season with salt and pepper. Transfer the arugula to plates, mound the couscous on top and drizzle with the remaining lemon oil. Top with the lemon slices and pistachios.