Heat 1 tablespoon oil in a medium skillet over medium-high. Add shallot, garlic, chile, lemongrass, and ginger and cook, stirring occasionally, until browned, 6–8 minutes. Stir in fish sauce, scraping up any brown bits. Stir in sugar and 1/2 cup water and bring to a boil; cook, stirring occasionally, until reduced by half, 12–15 minutes. Strain sauce into a medium bowl; discard solids. Let cool.