Prepare the mushroom rice first. Preheat a large frying pan over medium heat. Add the butter and allow it to melt, then add in the sliced mushrooms and diced onion. Sauté the mixture for 7 to 8 minutes, until the mushrooms are browned and the onion is translucent. Add the garlic and cook for another 30 seconds. Season the mushroom mixture lightly with salt, then transfer into a medium pot for cooking the rice.
To the pot, add the rice and chicken broth. Season the rice with salt, black pepper and just a sprinkle of ground coriander and paprika. Cover the pot with a tight fitting lid and bring the rice to a simmer, then reduce the heat to low and cook until the liquids are evaporated. Once the rice is cooked, fluff it with a fork and cool the rice completely.
For Butter Spread:
Next prepare the salmon spread: combine the softened butter with the salt, lemon zest, dill and smoked paprika. Mix until the seasoned butter is uniform.
Assembling the Coulibiac:
Using a sharp knife, remove the skin from the salmon and remove any bones. Trim the tail end of salmon and the sides, to make the salmon filet even sized. Ideally, the salmon should be about 6-inches wide. Keep the trimmed pieces and add them to the coulibiac also. Pat the salmon dry with a paper towel, then spread the herbed butter evenly all over.
Preheat the oven to 375F/190C and line a large baking sheet with parchment paper or silicone mat.
Begin assembling the pastry: roll out one sheet of thawed puff pastry into a 10x15-inch (25x38-cm) rectangle, or large enough to enough to have at least 1 to 1 1/2-inches around the entire fillet. Transfer the dough onto the lined pan. Spread about 1 ½ cups of the mushroom rice onto the pastry in the center, creating a bed for the salmon to sit atop of. Place the seasoned salmon on top of the rice, then top the fish a layer of sliced, hardboiled eggs. Watch my video recipe to see how it’s done!
Top the salmon with the remaining mushroom rice, forming and gently pressing it into a dome shape. Roll out the second sheet of puff pastry wider than the first, to cover the fillings comfortably. Brush the beaten egg onto the pastry around the salmon and rice, then add the second layer of dough. Use a fork to press the two layers together, sealing the fillings inside.
Roll out the third sheet of pastry the same as the second, then using a paring knife to cut 1-inch (2.5-cm) slits into the dough, in neat and overlapping rows. Gently pull apart the dough over the assembled pastry and it should reveal little circles. Seal the dough around the edges using a fork, then use a sharp knife to cut away the excess dough, leaving just ½ - 1-inch border all around. Brush the entire pastry generously with the egg wash.
Bake in the preheated oven for 45 to 50 minutes, until the pastry is a rich, golden color. Remove the pastry from the oven and let it cool for at least 15 to 20 minutes prior to serving. To serve, use a long, serrated knife to slice the pastry into thick slices. Serve with fresh lemon wedges and lemon sauce on the side.
For Lemon Sauce:
For the lemon sauce; this can be prepared while the coulibiac is baking. Heat a small frying pan over medium heat and add the butter. To the melted butter, add the diced shallot and cook for 3 to 4 minutes, over medium-low heat, until the shallot is tender but not browned. Add the lemon juice, white wine and salt and simmer the sauce for 4 to 5 minutes. Add the fresh herbs last, then remove the sauce from heat.