1. Preheat your oven to 220°C (425°F).
2. Thaw the spinach (or remove from the can) and let it drain in a colander.
3. In a small bowl, mix the goat cheese and sour cream with a fork. Gradually add milk until you reach a consistency similar to a "thin béchamel sauce". Season with pepper to taste. Add lemon juice.
4. In a square or rectangular baking dish, spread a thin layer of the cheese-cream mixture, then arrange the lasagna sheets.
5. Cover them with a thin layer of the goat cheese-cream mixture, then a layer of well-drained spinach (squeeze them with the palm of your hand if necessary).
6. Add a layer of smoked salmon slices and cover with lasagna sheets.
7. Continue layering until you run out of ingredients, only adding smoked salmon every other layer to avoid overpowering the taste of the spinach with its strong saltiness. The number of final layers will depend on the size of your dish!
8. Cover the last layer of lasagna sheets with the goat cheese cream, then a layer of thin slices of goat cheese log.
9. Bake at 200°C (400°F) for about thirty minutes; like a cake, insert a knife blade to check for doneness: when it's tender, it's cooked!