Melt the butter in a frying pan and add the mushrooms. Cook for about 5 mins, stirring occasionally, until they soften. Add the salmon to the pan and cook for 4-5 mins, turning until it’s cooked, but taking care not to overcook, or stir it too much, otherwise it will start to break down. Remove the pan from the heat. Stir the dill into the jar of Hollandaise sauce, then add about ¾ of sauce to the pan and stir lightly to coat the salmon.
Lay the pancakes flat on the work surface and spoon some salmon mixture down the centre of each. Grind some black pepper over the top and then fold each side of the pancakes to the centre, overlapping them. Carefully lift the pancakes and place them in the gratin dish. Spread the remaining Hollandaise sauce over the top.
Bake the pancakes for about 10-15 mins, or until they are hot. Remove from the oven and serve immediately with salad.
