Put a large pan of salted water on to boil. Peel and finely chop the shallots. Peel and crush or grate the garlic. Peel and grate the ginger. Finely slice the chilli, flicking out the seeds and white pith for less heat. Pop them all in a bowl. Finely grate in the7 lime zest. Set aside.
Trim the roots and dry tips off the salad onions. Finely slice them, greens and white parts. Slice the spring green leaves off their thick cores. Finely slice the leaves into thin ribbons.
The water in the pan should be boiling by now. Add 2 bundles of the Thai rice noodles and simmer for 3 mins, then drain them.
While the noodles simmer, put a wok or deep frying pan over a high heat for 1 min. Add ½ tbsp oil, the shallots, garlic, ginger, chilli and lime zest. Season with a pinch of salt. Sizzle for 1 min, then add the tomato sauce and cook, stirring for 1 min.
Add the salad onions and spring greens. Stir fry for 5 mins till the spring greens soften and have wilted down a little.
Add the crab meat to the pan. Stir fry for 2 mins to warm the crab through.
Toss the noodles into the wok. Use 2 forks to pick up and drop the noodles so they mix well with the crab, veg and sauce. Warm everything for 2 mins. Taste and add a little more salt if needed.
Divide the noodles between 2 warm bowls. Serve with wedges of the zested lime for squeezing.