For the pasta, place the flour, three of the eggs and the olive oil into a food processor and pulse until it forms small crumbs. Remove the mixture from the processor and pull together to form a dough.
Knead the dough lightly on a floured surface for 2-3 minutes until it is smooth and elastic. Wrap in cling film and place in the fridge for 20 minutes.
Flour the pasta machine. Starting at the lowest (thickest) setting, feed the dough through the machine, turning the handle with one hand and holding the dough as it comes through the machine with the other.
Change the setting on the pasta machine to the next-thickest setting, flour it again and feed the pasta sheet through the machine again, as before.
Repeat this process 3-4 more times, flouring the machine and changing the setting down each time - it helps to cut the pasta into smaller pieces as this will mean the pasta is less likely to dry out - any pasta you are not working on should be covered with cling film.
To make the tortellini, place both the crab meats into a bowl with a little juice, the paprika and parsley and mix to combine. Season to taste with salt and freshly ground black pepper and adjust the lemon juice accordingly.
Beat the remaining egg to make an egg wash. Lay a sheet of pasta onto the work surface and stamp out 6cm/2½in diameter discs. Place a spoonful of the crab mixture onto each disc and brush the edges of the pasta lightly with the egg wash.
Fold one half of the pasta over the filling then press lightly to seal. Pick the crescent of pasta up and twist the edges together, forming a little lip on the outside edge of the pasta. Repeat with the remaining pasta and filling then set the tortellini aside, covered in cling film, until ready to cook.
For the broth, bring a pan of salted water to the boil, blanch the parsley for 30 seconds then refresh in iced water. Drain, squeeze dry then roughly chop and set aside.
Heat a sauté pan until hot, add the butter and onion and cook gently for a few minutes until just softened. Add the peas and stock, bring to the boil then add the chopped blanched parsley.
Ladle the mixture in batches into a food processor and blend to make a fine broth - try a small amount first to see how much it comes up the sides when blending, be careful not to overfill as the broth is hot and can easily come out of the top. Strain into a clean saucepan then return to a simmer and check the seasoning.
Bring a pan of water to the boil, add the carrots, boil for one minute then add the leeks and cook for another two minutes until both are tender. Drain and season with a little olive oil.
To cook the tortellini, bring a large pan of salted water to the boil, add the tortellini and cook for 1-2 minutes or until they float to the surface then remove with a slotted spoon and drain. Toss with a little olive oil, salt, and freshly ground black pepper.
To serve, ladle the parsley and pea broth into a soup plate. Place a few tortellini on top then scatter the carrots and leeks around. Garnish with the chervil sprigs.