Add all ingredients except for the olive oil into the work bowl of a mini food processor. Process until the cilantro is very finely chopped and the mix has a light green color (about 1 minute). Scrape down the sides with a rubber spatula, if needed.: The first moment you combine the cream , buttermilk , roughly chopped cilantro , ground cumin , ground coriander , lemon juice , garlic salt , and salt in the bowl the room will start to smell herbaceous and tangy. You should notice the green color beginning to dominate the pale dairy as the herbs settle among the liquid. This mixture establishes the flavor base, so make sure the cilantro is fresh and the quantities are accurate. A mistake here is skipping the stems, which contain a lot of flavor, or over packing the bowl so the herbs do not process evenly.
With the blade running, slowly pour in the olive oil. Process until smooth.: As the processor runs, listen for a steady, even whir and watch the mixture transform from chunky to smooth and pale green. The scent will intensify, releasing citrusy herb oils that signal you are on the right track. This step helps the sauce develop uniform texture and prevents large herb bits that can make the sauce feel coarse. Avoid under processing or you will get uneven pockets of herb, and over processing for too long can warm the sauce and slightly change its brightness.
Scrape down the sides with a rubber spatula, if needed: After the initial processing, you may see unmixed herbs or dairy clinging to the bowl walls. Scraping them down integrates those trapped bits and ensures a consistent texture and flavor in every spoonful. The action also prevents dry pockets from remaining. The common mistake is skipping this, which leaves uneven flavor. Use the spatula to gather the mixture to the blades before the final emulsification.
With the blade running, slowly pour in the olive oil: Once the mixture is homogenous, keep the blade spinning and add the olive oil in a slow, steady stream. You will notice the texture change as the oil emulsifies into the dairy, creating a glossy, cohesive sauce that clings well. The sound will remain a smooth whir, and the visual cue is a slightly thicker, shiny body. Pouring too quickly will prevent proper emulsification and can cause the sauce to separate or remain oily rather than creamy.
Process until smooth: After the oil is incorporated continue processing for a few more seconds until the surface is uniformly glossy and the texture is silky. The aroma should be a balanced mix of herb, dairy tang, and a subtle warmth from the spices. This final refining step ensures any tiny pockets of herb or unblended dairy are fully combined. A frequent error is stopping too soon, which results in a less stable emulsion and a grainy mouthfeel; keep processing until you see a consistent sheen throughout.