In a large pot add the vegetable stock, salt and butter and bring to a boil. Stir in the couscous and cover the pot and let sit for about 15 minutes. The couscous will absorb all of the water, once this has happened stir in the tomatoes, feta and green onions and season with salt and pepper if needed. Transfer the couscous to a large serving platter and place the salmon fillets on top. Serve with wedges of lime.