Heat your grill for direct (over the coals) cooking and place your tomatoes and dried chiles close to the grill. Note: Mise en Place (everything in its place) is important with this recipe as you are working with high heat.
Using extra-long tongs, place your tomatoes on the grill
Turn the tomatoes once the skin begins to wrinkle and start to char, about 2-3 minutes each side.
Remove the tomatoes once they are browned on all sides, and set them in a bowl to rest. Please note that they will soften somewhat, but you don't want them to burst and lose all that delectable juice.
Place the dried guajillo chilis on the grill.
Watch the chilis closely and turn every 30 seconds until they start to turn a deep mahogany color (almost blackened). Note in the photo the difference between the colors (the top chilis are done).
Remove the chilis from the grill and place them in a bowl with 2 cups of water.
Place a bowl or other heavy object on top of the chilis so that they are submerged. We want the chilis to reconstitute and soften in the water.
After the chilis have reconstituted (about 30 minutes), drain and discard the water and set the chilis aside.
Gather your tomatoes, chilis, and dried ingredients together to make the sauce.
In order, place the tomatoes and chilis in the blender, followed by the salt, pepper, chili powder, cumin, and Mexican oregano.
Blend together until the mixture is mostly smooth and fully incorporated.
Heat the Dutch oven over a low-medium heat and pour the sauce from the blender into the Dutch oven.
Bring the sauce to a simmer and, using a slotted spoon, stir the sauce periodically to ensure the sauce doesn't stick to the bottom of the Dutch oven.
Cook the sauce for 20-30 minutes until it thickens and browns further. Immediately remove from the heat and let cool.
Use immediately or transfer to a covered container and refrigerate. The sauce can be refrigerated for 3-4 days (as if it will last that long once your eaters taste it!) or can be frozen for up to 2-3 months.