First make the pancakes. Sift the flour into a bowl, add the salt and sugar and make a well in the centre. Add the eggs to the well and beat with a wooden spoon, gradually drawing in the flour from the sides. Slowly add the milk, beating until you have a smooth, thin batter with a few small bubbles on the surface. Mix in the chives.
Place a pancake pan or a medium frying pan over a moderate heat and grease with a little butter. Add enough of the pancake batter to make a thin pancake, swirling it around the pan so it covers the base evenly. Cook for 1-2 minutes, until golden underneath, then turn and cook the other side. Turn the pancake out of the pan on to a piece of baking parchment and cook the remaining batter in the same way. The pancakes can be stacked on top of each other once cooked. (You will have more pancakes than you need, so either stop after you've made 4 and store the batter in the fridge for the next day, or treat yourself to the extra ones).
Remove any large stalks from the spinach leaves. Blanch the leaves for 10 seconds in a large pan of boiling salted water, then drain and immediately place under cold running water to stop cooking. When the spinach is cool, gently press out the excess water and lay the leaves out on a tea towel to dry.
Place a piece of cling film on the work surface and put a pancake on top. Place a spinach leaf on top of that, then a piece of sea trout. Season the fish well with salt and pepper. Wrap the spinach around the fish so it is fully covered. Brush one end of the pancake with some of the beaten egg yolk and wrap the trout up, ensuring the sides of the pancake are tucked in and the end sticks to the egg (it should look like a large sausage roll). Roll the whole thing up tightly in the cling film, taking care to keep the rectangular shape. Repeat with the remaining ingredients.