Thinly slice the bottom 4 inches of the stalk of lemongrass. Heat 1 tablespoon of the oil in a medium saucepan. Add the garlic, shallot, lemongrass and ginger and cook over low heat until softened, about 5 minutes. Add the shrimp shells and cook, stirring, until bright pink, about 3 minutes. Add the tomato paste and cook, stirring until glossy, about 2 minutes. Add the white wine and bring to a boil. Add the water and simmer over low heat for 30 minutes. Remove the shrimp stock from the heat and let stand for 10 minutes.