Rinse the Serrano peppers and cut the stems off.
Sauté the chopped onion and garlic in a hot pan.
Add the Mexican oregano, salt, and pepper. Stir in the cumin and keep on cooking until the onion starts to turn golden brown.
Add the water, vinegar, and Serrano peppers. Bring the whole mix to a simmer.
Add the mix to a blender and add the cilantro spring and juice of the lemon.
Blend until the desired consistency. It’s a good idea to keep the sauce a little bit chunky.
Strain the sauce through fine layers of cheesecloth or pour it directly into bottles. Store the sauce in a sealed bottle or a Mason jar and keep it in the fridge.