Prepare the chiles by removing the majority of the charred skin, cutting off the steam and seed pocket, and then cut down the middle of the chile to remove the seeds inside. If you want an extra spicy green chile sauce you can add in some of the seeds. I also like to keep a little of the charred skin to add flavor. Then slice the chile length wise into 1/2 cm strips and then cut width wise into small pieces.
Dice the onion and add to a pan over medium heat with the olive oil. Cook for 5 minutes until the onion is soft and translucent. Add the diced green chiles. Drain the can of rotel and then add to the pan, stir everything to combine.
Sprinkle the flour over the chiles, onion, and tomatoes. Stir until the flour is all absorbed roughly 1 minute. Slowly add a little water and stir until it makes a smooth paste. Add the remaining water and stir until all the flour is dissolved.
Simmer sauce over low-medium heat for 1 hour being sure to stir every now and then.
Add the garlic salt, salt, and cumin. Using a slotted spoon transfer half the green chile sauce to a food processor. Pulse until all smooth and then return to the pot. Stir until everything is combined.
Use right away to top your favorite dish or transfer to a container to keep in the refrigerator for up to 2 weeks. You can also freeze half the sauce to save for later.