Combine broth, lemongrass and 2 tablespoons shallot in a small saucepan. Cook over medium heat until reduced to 1/4 cup, about 10 minutes. Remove from heat and let stand for 10 minutes. Strain into a small bowl, discarding solids. Add remaining 2 tablespoons shallot, lime juice, soy sauce, fish sauce, sugar and chile oil; whisk to combine. Just before serving, stir in cilantro and mint.Advertisement
Make Ahead Tip: Cover and refrigerate for up to 1 day.
Vietnamese fish sauce is available in the international-food section of most supermarkets.
People with celiac disease or gluten-sensitivity should use soy sauces that are labeled "gluten-free," as soy sauce may contain wheat or other gluten-containing sweeteners and flavors.