Drain jalapeños, reserving brine.
Add oil to small pan over medium heat.
Add chopped onion and sauté 5 minutes or so until soft
Once the onion has softened add garlic, jalapeños, oregano, cumin, and black pepper.
Add 3/4 cup brine and 1/2 cup water. Bring to a simmer.
Once the mixture starts to simmer remove from the heat.
If using an immersion blender, add 8-10 sprigs cilantro and the juice of 1 lime to the and blend thoroughly. If using a blender, carefully transfer mix to a blender, add remaining ingredients and blend.
Once blended, add xanthan gum, if using and blend again and pour into mason jar or squeeze bottle and cool.
Store covered in the refrigerator.
Sauce will last for about a month in the refrigerator.