Char the Ingredients: Heat a comal or cast iron skillet over medium-high heat. Place tomatoes in the center, then onions, garlic, and finally chiles on the outer edge. Toast the arbol chiles for just a few seconds until fragrant — don’t burn them. Transfer toasted arbol chiles to a bowl and cover with boiling water. Let steep 20 minutes until soft. Cook remaining ingredients until blackened and tender, flipping as needed. Set aside separately.
Build the Salsa in the Molcajete: Rinse your molcajete with cool water. Add garlic, a pinch of salt, and some chile steeping water (if not bitter). Mash until smooth. Add the rehydrated arbols and fresh serranos. Mash until smooth, adding water as needed. Add onions and mash until mostly smooth. Add tomatoes and mash to your desired consistency. Season with salt and bouillon to taste.
