01 -
Set a dry comal or heavy cast iron skillet over medium-high heat with no oil. Let it heat for 2 to 3 minutes until properly hot before anything touches it.
02 -
Place the tomatoes, tomatillos, serrano chiles, onion slice, and unpeeled garlic cloves directly on the hot surface. Roast for 8 to 12 minutes, turning every 2 to 3 minutes, until all sides are deeply charred and blistered. The tomatoes should collapse slightly and the tomatillos should turn a dull olive color and feel soft when pressed.
03 -
Let everything cool for 2 minutes. Slip the garlic out of its skin. Remove the stems from the chiles. Keep the seeds for more heat or discard them for a milder salsa.
04 -
Transfer all roasted ingredients to a blender with the cilantro and salt. Pulse 3 to 4 times for a chunky, rustic texture, or blend longer for a smoother consistency.
05 -
Add lime juice if using and blend briefly to incorporate. Taste for salt and heat. If the salsa is too thick, loosen it with a tablespoon of water at a time until it reaches the consistency you want.