Preheat the oven to 400°F. Heat the oil in a pan and sauté the onion for 5 mins. Add the frozen spinach and 3/4 cup water and simmer gently for 20-25 mins until heated through and most of the liquid has evaporated. Season.
Meanwhile, make the béchamel sauce. Melt the butter then add the gluten-free flour and cook for 1 min. Gradually whisk in the stock, bring to a boil then simmer for 5 mins until thickened. Whisk in the sour cream and dill and season to taste.
To build the lasagna, layer the spinach, béchamel sauce, salmon strips, and lasagna sheets in a shallow ovenproof dish, ending with a layer of salmon and béchamel sauce.
Distribute the cheese on top then bake in the oven for 40-45 mins until golden and bubbling.