Put a 4-quart pot of water to a boil and prepare an ice bath. Once boiling, add the shucked fava beans and blanch for 3 minutes. Drain and shock in an ice bath or rinse under cold water. When cool, peel the outside skin from the favas to reveal the brightly colored bean inside. Set aside.
Bring the stock and 2 cups water to a boil in another 4-quart pot, and then reduce to a simmer. Add the couscous and saffron. Cook like pasta until the couscous is al dente, 6 to 8 minutes or according to your package instructions. Drain the couscous, reserving the broth and set both aside.
Meanwhile, fill a medium bowl with water. Slice the leek in half lengthwise. Slice the leek halves into half-moons about 1/3-inch thick using only the white and light green parts. Put the slices into the bowl of water and rinse to clean the leeks, allowing any grit or sand to collect on the bottom of the bowl. Scoop the leeks out of the bowl of water and set aside.
Preheat a 12- to 14-inch skillet over high heat. Add 2 tablespoons of the olive oil. Sprinkle both sides of the fish fillets generously with salt and pepper. Sear the fish, skin-side down, until the skin is crispy and the fish is about two-thirds cooked, 6 to 8 minutes. Flip the fish and finish cooking on the flesh side for about 3 minutes.
Meanwhile, reheat the saffron broth to a simmer. Heat the remaining olive oil in a small saute pan and saute the leeks until soft, 2 minutes. Add the favas and a pinch salt and cook until warmed through. Add the tomatoes and remove from the heat.
To serve, spoon the couscous into the center of 4 shallow soup bowls. Top with the seared fish. Sprinkle the sauteed favas and leeks around and spoon the saffron broth over top of the vegetables.