For the pistou: Combine the oil, basil, lemongrass and chile in a mortar and pestle or food processor and crush together.
For the soup: Heat the olive oil in a large pot. Add the garlic and onions and cook gently, stirring occasionally, until soft and translucent. Add the tomato puree, bay leaves, carrots, courgettes and thyme and cook until the vegetables are al dente (tender but still a little crunchy), 15 to 20 minutes. Add the boiling water and green beans and return to a boil, and then add the pasta. Cook until the pasta is al dente, 10 minutes.
Stir in the peas and white beans. Remove the sprig of thyme and the bay leaves, and then add the salt and sugar and black pepper to taste.
Serve immediately in bowls, with a dollop of Vietnamese pistou on top of the soup.