Start by roasting the tomatoes in a 400F oven.
Use a damp towel to wipe off any dusty crevasses on the dried chiles. To de-seed them it's easiest to cut off the stem and make a slit lengthwise, then use your hands to pull out the ribs and seeds.
Saute 1/2 onion and 3 whole garlic cloves in a dollop of oil over medium heat. When the onion starts to soften add the dried chiles. (It's worth cutting the chiles into smaller pieces before adding them to the pan.)
Let the chiles cook for a few minutes and then add the roasted tomatoes to the pan along with: 1/2 teaspoon Mexican oregano, 1 teaspoon salt, freshly cracked black pepper, 2.5 cups water. (I also add 1 chipotle in adobo and 1 tablespoon of adobo sauce but this is optional.)
Simmer for 5 minutes or so and then add everything to a blender and combine well.
Strain the sauce over a mixing bowl and discard the leftover solids. You might have to use a spoon or spatula to push the sauce through the strainer.
Taste for salt level. I added another pinch of salt along with a splash of cider vinegar.
Serve immediately or store in an airtight container in the fridge (in a squeeze bottle!).