Pop the peanut butter in a bowl and stir in 1 tbsp of the tamari. Peel and finely chop the garlic. Halve the chilli and scoop out the seeds and white pith. Finely chop the chilli. Add the garlic and chilli to the bowl. Finely grate in the lemon zest and squeeze in the juice from one half. Stir together, adding 2 tbsp cold water to loosen the texture to a pouring consistency.
Peel the carrots and slice them thinly on the diagonal. Finely chop the coriander stalks, setting the leaves aside for later. Slice the mushrooms into 1cm-thick pieces. Slice the leaves off the radishes and discard them. Rinse the radishes and shake them dry, then halve or quarter them – or leave them whole if they're very little.
Put a large, covered pan of salted water on to boil. Trim the roots and top 1cm off the spring onions and slice them thinly on the diagonal. Roughly chop the reserved coriander leaves.
Set a wok or deep frying pan over a medium-high heat for 2 mins. Add 1 tbsp oil along with the chopped carrots, mushrooms, radishes and coriander stalks. Stir fry for 3-4 mins till golden and just cooked. Take off the heat and pop on a lid, or cover with a baking tray, to keep the veg warm.
The pan of water should be boiling by now. Add 2 bundles of the noodles. Simmer for 3-4 mins till tender, then drain and return to the pan. Pour in the peanut sauce and stir well to coat the noodles.
Add the noodles to the veg in the wok and toss together over a medium heat for 2 mins.
Pile the noodles and veg into 2 warm bowls. Scatter over the spring onions and coriander leaves. Serve with wedges of the remaining lemon.