Bring cast iron pan to medium-high heat and add a splash of oil.
Add the whole jalapeño peppers, quartered onion, and garlic to the hot pan.
Roast ingredients for about 5 minutes until lightly browned, turning frequently for even searing.
Remove the peppers and carefully slice them into rings.
Simmer the jalapeños and onions in a pan with the water for about 5 minutes, stirring occasionally.
Add in the garlic, salt, cumin, and sugar, stirring to combine. Simmer an additional 1 minute or so.
Add the simmered mixture into a blender along with the cilantro and vinegar.
Blend until smooth and let cool.
Season to taste by adding more salt and/or cumin, as desired.
Add into hot sauce bottles, mason jars, or container of choice and store in the refrigerator.