Heat a comal or cast iron skillet over high heat. Add the tomatoes, onion segment, jalapeño, and garlic and dry roast, flipping every 30 seconds, until softened and charred on all sides. Transfer to a cutting board.
Roughly chop the onion and slice the tomatoes in half.
Peel the paper off the garlic and add it to the molcajete along with a small pinch of salt. Grind it in the bowl of the molcajete until it turns into a thick paste.
Add the jalapeño and grind it to your desired consistency. Do the same with the onion and tomatoes.
How to adjust the spice level. If your jalapeño turns out super spicy, you can bring down the heat by mashing an additional roasted tomato or adding chunks of avocado (like in my salsa Mexicana). If it’s not spicy enough, add in a dry roasted serrano pepper or one chile de árbol.
Don’t peel the charred skin. Many recipes require you to peel the charred skin off your vegetables, but that is what gives this molcajete salsa its signature rich and smoky flavor.
Don’t have a molcajete? You can mash this salsa in a large bowl using a fork, hand masher, or even the bottom of a cup for a similar texture. You could also pulse in a blender or food processor until it reaches your desired consistency.