Bring a large pot of water to a boil over high heat. Add the chiles and let cook 2 to 3 minutes, then drain and cool. Wearing clean kitchen gloves, cut off and discard the stems and set aside the chiles.
Heat the oil in a skillet pan over medium-high heat, then fry the shallots until translucent. Add the tomatoes and bring to a simmer, then turn off the heat and let cool.
Pour the tomato mixture into a blender and add the chiles, vinegar, fish sauce, sugar and garlic and blend until smooth. Transfer to a clean jar or bottle and store in the fridge for up to 10 days.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.
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