Zest lemon and reserve the zest. Cut the lemon into 8 slices. Season salmon with salt and pepper and spread 1 1/2 teaspoons pesto on each piece.
Heat 1 tablespoon oil in a large skillet over medium-high heat. Add half the fennel; cook until brown on the bottom, 2 to 3 minutes. Transfer to a plate. Reduce heat to medium and repeat with the remaining 1 tablespoon oil and fennel. Transfer to the plate. Add couscous and scallions to the pan; cook, stirring frequently, until the couscous is lightly toasted, 1 to 2 minutes. Stir in broth, olives, pine nuts, garlic, the reserved lemon zest and the remaining 2 tablespoons pesto.
Nestle the fennel and salmon into the couscous. Top the salmon with the lemon slices. Reduce heat to medium-low, cover and cook until the salmon is cooked through and the couscous is tender, 10 to 14 minutes. Garnish with fennel fronds, if desired.
