1 Pile the flour and the semolina onto the work surface. Create a well in the middle and crack the eggs into it. Add the olive oil and half a tsp salt. Knead into a smooth, elastic dough that comes away cleanly from the work surface. Adjust the amount of flour if necessary. Wrap in cling film and chill for around 30 minutes.
2 Split the dough into portions to make it easier to work with and roll them out on a floured work surface. Leave to rest on the work surface until you need them.
3 Heat the oven to 200°C (180°C in a fan oven), 400°F, gas 6 and grease a baking dish with butter.
4 Fry the onion in hot butter and add the spinach allowing it to wilt. Add the tomatoes and stir in the ricotta. Season with salt and ground black pepper and remove from the heat.
5 Add two thirds of the herbs to the pot and mix well.
6 Cut the pasta dough to fit the baking dish and put the lasagne together by alternating between the ricotta and spinach mixture, the salmon and the pasta dough. Start and finish with the pasta.
7 Top the lasagne with the cheese and the remaining herbs and bake for around 40 minutes.