Heat coconut oil in a large Dutch oven over medium heat. Once the coconut oil has melted, add the onion, garlic, ginger, lemongrass, red curry paste, and sauté together for 1-2 minutes, just until the onions and garlic become soft and fragrant.
Add the chicken and cook together, stirring regularly, until all sides of the chicken pieces are lightly browned (about 5 minutes).
Pour in the chicken broth, increase heat to medium-high and bring to a boil.
Lower heat to medium, then pour in the coconut milk and add the mushrooms.
Simmer on medium until the chicken is cooked through and a thermometer inserted into the center of one of the larger pieces of chicken reads 165°F.
Turn off the heat, then add the fish sauce, lime juice, and soy sauce.
Serve garnished with cilantro and green onions.
Notes
Makes approximately 6-8 servings. Heat level: this recipe makes what might be considered medium spicy soup. Reduce the amount of Thai red curry paste to one teaspoon (or even less) if you prefer it to be mild. Storage: store in an airtight container in the refrigerator for up to 3 days.