Preheat the oven to 400°F/200°C. Heat the oil in a saucepan over medium heat. COOK the garlic, lemon zest and thyme for 2 minutes. ADD the flour and cook 1 minute, stirring to combine. REMOVE from the heat and gradually add the milk, stirring constantly, then return to the heat and cook, stirring, until thickened. ADD the salt, pepper, Dijon mustard and ¾ cup of the combined Parmesan and Gruyère cheese (reserving ¼ cup) and stir until the cheese melts. REMOVE from the heat and stir in the parsley and lemon juice. GREASE an 8-inch (20cm) square baking dish lightly with olive oil. PEEL the potatoes and slice very thinly (using a mandoline or slicing attachment on a food processor is easiest). PLACE one sliced potato in a layer on the bottom of the baking dish, overlapping slightly. ARRANGE one third of the sliced leek and salmon evenly on top. SPOON over one quarter of the cheese sauce. REPEAT the layers with the remaining potatoes, leek and salmon, finishing with the final layer of potato and topping with the last quarter of cheese sauce. SPRINKLE with the reserved ¼ cup of grated cheese. COVER with foil and bake for 45 minutes, then remove the foil, and bake for a further 30 minutes. ALLOW the gratin to stand 10 minutes before serving.