In a wide pot, heat 2 tablespoons of coconut oil over medium high until very hot. Add the onion, garlic, ginger, lemongrass, and chili peppers and cook for 7 minutes, stirring frequently, until beginning to brown.
Add the chicken stock and bring to a boil. Stir in the fish sauce and coconut sugar. Boil for 2-3 minutes, then reduce heat to low and simmer, uncovered, for 30 minutes. The broth should reduce by about 1/3. (Do not let the broth reduce more than that. If the broth is starting to reduce too much, add a bit of water and cover and continue simmering.).
Taste and season according to your tastes. Strain the broth through a fine mesh sieve into a large bowl. Discard the solids and wipe out the pot. Transfer the broth back to the pot.