Prepare the Ingredients: Begin by removing the stems from the fresh Thai chilies and peeling the garlic cloves. Place the chilies, garlic, salt, light brown sugar, and vinegar into a food processor. Process the mixture until the ingredients are finely chopped and well combined.

Cook the Chili-Garlic Mixture: Heat ½ cup of neutral oil in a heavy-bottomed pan over medium heat. Once the oil is warm, add the processed chili-garlic mixture to the pan. Increase the heat to medium-high and cook the mixture for 8 to 9 minutes, stirring frequently to prevent burning. The sauce is ready when it transitions from a runny, pink consistency to a thick, rich red paste, indicating that the water content has evaporated.

Incorporate the Remaining Oil: Once the mixture has thickened, add the remaining 1 cup of neutral oil to the pan. Stir thoroughly to combine, ensuring the sauce achieves a smooth and glossy texture. Turn off the heat and allow the sauce to cool completely.

Store the Sauce: Transfer the cooled chili garlic sauce into a clean, airtight jar. If you do not plan to use the sauce within a few days, store it in the refrigerator to maintain freshness. Properly stored, the sauce can last for several weeks.