Put the potatoes in a pan of lightly salted water, bring to the boil and cook for 15 minutes or until tender. Drain and set aside. (This can be done well ahead.)
Meanwhile make the paste: put all the ingredients in a food processor. Add some of the oil of the jarred sundried tomatoes. Process until you get a coarse paste, then transfer the paste to a bowl. (This can be done ahead too).
Put the fish fillets onto a plate. Add a dollop of the paste on top of them, then rub them well. Set aside.
When the potatoes have cooled, cut them into 1 cm thick slices. Cut the courgette and onions too into 1 cm thick slices.
Preheat the oven to 180°C.
Oil a baking dish, then line it with the onion slices. Arrange the potato and courgette slices on top, slightly overlapping each other. Season with salt and pepper, then brush them with the paste. (You might be able to do another layer).
Put the dish in the oven and bake for about 20 minutes until the veggies are golden.
Take the dish out of the oven.
Place the coated fish on top of the veggies, scatter the olives around, then drizzle with some oil & lemon juice.
Bake for 5-8 minutes (depending on the thickness of the fillet) until the fish has just cooked through (don’t overcook it).
Serve warm, with dill or other chopped green herbs scattered on top.