Working in batches, wilt the spinach in a large skillet over medium heat. Transfer the spinach to a bowl lined with cheesecloth.
When the spinach is cool enough to handle, squeeze out the excess moisture. Discard the liquid and place the drained spinach in the bowl.
Sauté the onion and transfer the onion to the bowl with the spinach. To the spinach and the onion add the scallions, parsley, dill, and feta.
Stir to combine and taste. Add additional salt accordingly. Stir in the couscous followed by the egg and set the mixture aside.
Season the salmon on both sides with the salt, sugar, and lemon zest. Set it aside as well.
Arrange a sheet of phyllo dough on the baking sheet. Brush it all over with some of the butter. Place the next sheet on top until all 8 sheets are stacked on top of each other.
Place the salmon in the center of the pastry. Place the spinach filling on top of the salmon and fold all the edges of the phyllo dough towards the center.
Brush the surface with the remaining butter and sprinkle with sesame seeds. Transfer the wellington to the oven and bake for 30 minutes, rotating the pan halfway through.