
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.
Season the salsa with salt to taste and serve.