Finely chop the garlic and sauté in the butter, add spinach and let it wilt, then let it cool. Roll out the puff pastry and spread the mustard in the middle in a more or less rectangular shape, place the raw salmon fillets side by side on top in a long rectangle and season with pepper and salt. Squeeze the spinach a bit and distribute it on top of the salmon, and then add the cheese, keeping a tablespoon of cheese aside to sprinkle on top later.
Now cut the 2 sides of the dough into strips of 2 cm and alternately cross them over the whole in a stair-step manner, making sure that you can hardly see any filling. Brush with beaten yolk and a splash of milk, and sprinkle with the cheese and sesame seeds.
Bake for 30 min in a preheated oven at 185°C or until it looks nicely golden brown. Let it cool slightly and cut slices with a bread knife.