Heat a wok over high heat until smoking, add the canola oil, and give it a swirl. Add the lemongrass, and toss for a few seconds to release its aroma. Add the chicken, and sear for a few seconds, then cook, tossing, for 2—3 minutes until almost cooked through. Add the hot vegetable stock, coconut milk, lime leaf, tomatoes, and mushrooms. Season with the fish sauce, lime juice, and superfine sugar and bring to a boil. Then turn the heat down to medium, and simmer for another 2 minutes. Remove from the heat, garnish with cilantro, and serve.