Place a comal or cast iron skillet over medium-low heat. Toast the chiles de árbol, turning continuously for 15–20 seconds or just until fragrant. Be careful not to burn them—remove immediately when ready.
Increase the heat to medium. Roast the tomatoes, tomatillos, and 2 unpeeled garlic cloves until charred on all sides, soft and cooked through. Some ingredients may finish roasting sooner than others; remove each as it’s ready.
Peel the roasted garlic and add it to the molcajete along with the raw peeled garlic clove and 1/2 tsp of kosher salt. Grind into a smooth paste.
Next, add the toasted chiles de árbol and grind them down as finely as possible. Mash the tomatillos and tomatoes one at a time until completely broken down.
Add salt to taste—I used an additional 1/2 tsp of kosher salt.
Lastly, mix in the diced white onion and chopped cilantro with a spoon. Serve and enjoy!