Step 1: Preheat oven to 400°F (200°C). Season salmon with 1 tsp olive oil, salt, and pepper, then bake on a sheet for 12–15 minutes until flaky; discard skin and flake meat into large chunks. Lower oven to 375°F (190°C).
Step 2: Sauté the onion and half the minced garlic in 1 tbsp olive oil over medium heat. Add spinach in batches until wilted, let cool slightly, squeeze out all excess moisture with a clean towel, and rough-chop.
Step 3: In a bowl, mix the dried spinach with the ricotta, 1 cup of mozzarella, 1/4 cup of Parmesan, nutmeg, salt, and pepper.
Step 4: Melt butter and 2 tbsp olive oil in a saucepan, cook the remaining garlic for 1 minute, then whisk in flour for 2 minutes. Slowly pour in the milk and heavy cream, whisking constantly until thickened, then stir in the Italian seasoning and 1/2 cup of Parmesan before removing from heat.
Step 5: Grease a 9x13-inch baking dish. Assemble layers twice in this order: a thin layer of cream sauce, 3–4 lasagna noodles, half the spinach mixture, half the flaked salmon, a drizzle of cream sauce, and a handful of mozzarella. Finish with a final layer of noodles topped with the remaining cream sauce, mozzarella, and extra Parmesan.
Step 6: Cover loosely with foil and bake at 375°F (190°C) for 25 minutes. Remove the foil and bake uncovered for an additional 15 minutes until bubbling and golden-brown. Let rest for 15–20 minutes before slicing.