Rinse the Hatch chiles and roast them in the oven at 400F for 30 minutes or so. You can alternatively roast them on the grill.
As the chiles roast you can start building the rest of the soup. Add 3 tablespoons butter to a saucepan over medium heat. Roughly chop an onion and add it to the saucepan along with 4 peeled garlic cloves. Snce we're blending all of this eventually I usually just leave the garlic cloves whole.
Once the onion has softened you can add 3 tablespoons of flour to the mixture. Stir well until the flour absorbs most of the liquid -- this step will help thicken up the soup but you can consider it optional if you want to avoid flour.
Add 4 cups of stock along with 1 tablespoon Mexican oregano, 1/4 teaspoon cumin (optional), 1/2 teaspoon salt, and freshly cracked black pepper. Simmer for a few minutes or until the chiles are fully roasted.
Once the roasted chiles have cooled a bit you can pull off as much skin as you can, but don't worry about getting rid of all of it. You can also de-stem and de-seed the chiles at this point.
Add the Hatch chile pieces to your soup mixture and bring everything up to a uniform temp. Then transfer everything to a blender and combine well. Be sure to use caution when blending hot liquids -- I usually just hold a paper towel over any openings just in case you get a geyser.
Return the blended soup to the saucepan. Simmer for 5-10 minutes and be sure to do lots of taste testing! I added more salt to this batch (about 1.5 teaspoons total) but keep in mind the salt level will depend on which stock you use. I also added another generous pinch of cumin to this batch. At this point I also added 1 can of corn (drained and rinsed) but this is optional.
Serve immediately but don't forget to garnish! I used finely chopped cilantro stems and a couple dashes of Valentina hot sauce. A squeeze of lime and Mexican Crema are good options as well.
Store leftovers in an airtight container in the fridge where it will keep for a few days.