Dice the onion and chile and mince the garlic. Heat a medium pot over medium heat and add 1 tablespoon of the olive oil. Stir in the onion, chile, and salt, cooking until the onion is soft, 8 minutes or so.

Stir in the garlic, cooking until fragrant, about 1 minute, then add the lentils and water. Bring to a boil, reduce to a simmer, and let cook until the lentils fall apart, about 20 minutes. The mixture will look relatively thin—I assure you it will thicken as you blend.

Transfer the mixture to a high-speed blender or food processor. Add the vinegar and remaining 2 tablespoons olive oil, then puree until smooth. If the puree is too thick, add a splash or two of hot water. It will continue to thicken as it cools. If the puree feels too thick to spread, add a bit more water and puree once more. Taste and adjust the vinegar and salt to your liking before serving.