Peel and finely dice the onion. Trim and finely dice the celery and fennel. Peel and grate or crush the garlic. Pare or zest the lemon. Juice it.
Cut the pork strips in to bite-size chunks. Heat 1 tbsp olive oil in a large pan. Add the pork. Cook and turn for 4-5 mins till golden. Lift out of the pan with a slotted spoon and into a bowl.
Add more oil if required and throw in the veg. Add the fennel seeds. Cook on a low heat for 10 mins till soft. Add the garlic and pork.
Crumble the stock cube into a jug and pour over 400 ml boiling water. Pour into the pan. Tip in the lemon juice. Cover. Cook on low heat for 20 mins. Stir regularly.
While the pork’s cooking, tip the spaghetti into a large pan with a pinch of salt. Pour in enough boiling water to cover it. Bring to the boil and simmer for 10 mins till al dente (cooked but a little chewy).
Peel long ribbons from the courgette, discarding the seeded core (or keep in the fridge for soups or stocks, or add it to your compost).
Strip the leaves from the tarragon and discard the stems. Once the pork has cooked, pour in the crème fraîche. Stir. Add the courgette ribbons and most of the tarragon. Keep on a low heat.
Drain the spaghetti, saving a cupful of the water. Add the spaghetti to the pork and toss together. If the sauce is too thick, add drops of the pasta water till you have the consistency you want. Garnish with lemon zest and the tarragon leaves to serve.