How to make it
Combine tomatillos, poblanos, jalapeños, garlic, brown sugar, and salt in a food processor. Pulse until combined well but still chunky. Pour into a glass jar, cover with cheesecloth and keep at room temperature for 3 days, stirring once a day.
Transfer chili mix into a blender. Add vinegar and cilantro and blend until smooth. Transfer puree to a pot and bring to a boil. Reduce heat and simmer for 15 minutes.
Blend mixture until smooth. Strain through a strainer lined with cheesecloth. Sauce may separate over time, so shake well before using.