For the salmon pâté, wash the spinach with cold water and drain well. Remove any bones from the salmon fillet (use tweezers), acidify with lemon juice and season with salt.
Peel and finely chop the onions and sauté in hot butter until golden. Add the spinach and let it fall together. Season with salt and pepper.
Butter a loaf pan (about 25 cm long) and line with the puff pastry, leaving a generous rim hanging over. Squeeze spinach well and place about half in the loaf pan. Now place the salmon fillet on top and finish with the spinach. Fold the puff pastry over the filling and glue the seam together with a little water. Bake the salmon pâté in a preheated oven at 225 °C for about 40 minutes.