Optional - Heat a small skillet over medium heat. Add the cumin seeds and toast, stirring and shaking the skillet often, for 1-2 minutes. The seeds will deepen in color and become highly aromatic. Remove from heat and allow to cool slightly. Then use a mortar and pestle to grind to a powder. Set aside.
Add the cilantro leaves, mint leaves, Serrano, garlic clove, ground cumin, salt, lemon juice, and 2 tbsp water into a (preferably small) blender. Blend, scraping down the spice grinder/blender as needed, until it reaches a fine consistency. If needed, add another tablespoon or two of water to make it easier to blend. It should become a runny paste, with the leaves no longer visible.
In a small bowl, whisk the yogurt until smooth and no longer grainy. Stir in the crushed green paste. Taste and add salt or more lemon juice, if desired. For a thinner, restaurant-like consistency, add ½-1 tbsp of water. See notes for how to store.
Video
Notes
Storing: This dip keeps well, which is why restaurants love it!
To refrigerate: Store in an airtight container in the fridge for up to 5 days.
To freeze: Freeze before adding yogurt. After grinding the herbs (Step 2), place in an airtight bag or container and freeze. When ready to prepare, thaw the frozen solid at room temperature (don't microwave!). Then add it to the whisked yogurt (Step 3).
Variations:
Feel free to play around with the cilantro & mint ratios. I like much less mint than cilantro because I find too much mint can get quite strong and bitter.
Substitute lime juice instead of lemon for a slightly different flavor.
Feel free to add any spices or ingredients you'd love here. See post for optional add-in ideas.