Peel and finely chop the onion. Trim the dry ends off the celery and finely slice it. Trim and finely chop or coarsely grate the courgettes.
Warm a medium pan over a medium-low heat for 2 mins, then add 2 tsp olive oil and the veg. Season with a pinch of salt and pepper. Pop a lid on the pan and sweat the veg for 10 mins, stirring once or twice, till the veg have started to soften.
While the veg sweat, crumble the stock cube into a heatproof jug and stir in 800ml boiling water to dissolve it. Finely grate the zest from the lemon. Run your fingers down the thyme sprigs to pick off the leaves.
Stir most of the lemon zest and thyme (keep a pinch of each back for garnishing) into the softened veg. Stir in the risotto rice. Cook and stir for 2 mins.
Add a splash of vegetable stock, around 100-150ml, to the pan. Cook, stirring a few times, till all the stock has been absorbed. It should take around 25 mins. If it's cooking more quickly, turn the heat down a little.
Flake in the crab meat and add the final splash of stock. Cook and stir for 5 mins till the stock has been absorbed and the rice is tender.
Squeeze in the juice from half the lemon. Taste the risotto and add a pinch more salt and pepper if you think it needs it. Ladle the risotto into 2 warm bowls. Scatter over the last few thyme leaves and a pinch of lemon zest. Serve with wedges of the remaining lemon for squeezing.