To make the pasta, soak the saffron in 1 tablespoon of boiling water for 5 minutes.
Place all the pasta ingredients, including the saffron in its water, in a food processor and pulse until the dough comes together. Turn it out onto a lightly floured work surface and knead well, until smooth. Divide the dough into four pieces, wrap each piece in cling film and refrigerate for 2 hours.
Cut each piece of dough in half. Flatten one piece on a lightly floured surface. Roll it through a pasta machine, gradually making your way to two notches from the thinnest setting. Lay the pasta on a work surface and cut out a 7.5cm/3in disc. Repeat this step with the remaining dough, you should have eight discs. Allow the pasta to dry out on the surface while you make the sauce.
Preheat the oven to 200C/180C Fan/Gas 6.
To make the sauce, fry the shallot in the oil over a medium heat until softened. Add the lemon peel and vermouth and bring to the boil. Simmer until reduced by half. Add the cream and Gruyère, bring back to the boil and simmer until thickened. Pass through a sieve into a clean saucepan and set aside.
Drop the pasta discs into a pan of simmering salted water. Cook for about 3 minutes, then lift out with a slotted spoon. Season and drizzle over a splash of olive oil.
Heat a frying pan over a medium heat, add a splash of rapeseed oil and cook the spinach until just wilted. Season with salt and pepper.
Lightly oil a large baking tray and lay on four pasta discs. Divide the spinach between the discs and top with the crab meat and two slices of tomato. Finish with another pasta disc and spoon over the Gruyère sauce. Bake for 10–15 minutes, or until bubbling and lightly browned. Serve immediately on warmed plates.