Remove the stems, seeds, and veins of the guajillo chiles. In a cast-iron skillet over medium, dry toast them for ~30-60 seconds per side, or until fragrant.
Add the guajillos to a pot of water. Bring it to a boil, then turn the heat off, cover the pot, and let the chiles rehydrate for 10-15 minutes until softened.
Heat olive oil in the same cast-iron skillet over medium. Add in diced onions and sauté for 3 minutes, or until translucent. Add in minced garlic and cook for another 1-2 minutes.
Drain the guajillo chiles (saving the water) and transfer them to a blender along with the onion mixture. Add sea salt and cumin and mix on high until smooth, adding soaking water or vegetable broth as needed until it blends.
Add a little more olive oil to a pot heated over medium, and strain the sauce in. Season for 5-10 minutes, or until the color deepens. Taste and adjust salt to your liking. Serve immediately, or let it chill in your fridge for a few hours. Happy eating!