Parboil the potatoes for 8 minutes, then drain and set aside to cool. Dry the pan and return to medium heat, adding the oil. Saute the onion for about 7 minutes, stirring, until softened. Add the garlic, cook for 1 minute, then add the spinace and cook, stirring until just wilted down. Tip into a deep 1/3 litre baking dish and spread the mixture across the base.
Pre-heat the oven to 220*C. Place the salmon strips neatly in 1 layer across the spinach mixture. In a bowl, mix the cream, lemon zest and dill or parsley, seasoning well. Pour all over the salmon and spinach and onion base.
Remove the peel from the potatoes with your fingers, and coarsely grate the potatoes directly onto the creamy salmon in a craggy layer. Drizzle with a little olive oil, season to taste and place on a baking sheet. Bake for 30 minuttes, or until it's bubbling and the potato is a deep golden brown and crispy.