Preheat the oven to 180°C (360°F / Gas 4).
Bring a large pot of lightly salted water to a boil. Blanch lasagne sheets one at a time for about 1 minute, drain and lay flat on a clean kitchen towel to dry.
Make the ricotta sauce by blending the ricotta with a splash of milk or water until smooth and creamy; season with salt, white pepper and a pinch of nutmeg. Adjust liquid to reach a pourable but thick consistency.
Heat a large skillet over medium-low heat and add a glug of extra virgin olive oil. Sauté the crushed garlic and chopped chili (if using) briefly until fragrant.
Add the sliced leek and cook gently until softened, about 3–4 minutes.
Stir in the spinach and chopped kale, cover and cook 2–3 minutes, then uncover and sauté until the greens are fully wilted; add a little water if the pan gets too dry.
Halfway through cooking the greens, add the cubed salmon and cook until the salmon turns pale pink and is cooked through, breaking it into bite-sized pieces as it cooks.
Season the vegetable and salmon mixture with salt, white pepper, lemon zest and a pinch of nutmeg; stir to combine and remove from heat.
Lightly oil the bottom of a large ovenproof dish and spread a little of the ricotta sauce over the base.
Layer lasagne sheets over the sauce, then add half of the cooked vegetable and salmon mixture, sprinkle half the chopped chives and parsley, and spoon over more ricotta sauce. Repeat with remaining sheets, filling and herbs.
Finish with a top layer of ricotta sauce and evenly sprinkle the grated parmesan over the surface.
Bake for 20–25 minutes at 180°C (360°F) until the top is golden and the lasagna is heated through; let rest a few minutes before serving.