In a large saucepan, heat the oil over medium-high heat. Add the green curry paste, lemongrass, and garlic and stir until aromatic, about 1 to 2 minutes. Add the sugar and fish sauce and cook, stirring, for 2 to 3 minutes, until natural oils start to come to the top of the sauce. Add the coconut milk and whisk until combined, then add the chicken stock and continue to whisk for about 30 seconds more. Once the sauce begins to boil, reduce to a simmer and reduce for about 10 minutes, stirring every few minutes.