Heat a grill to medium high heat. Place the whole tomatoes, onion, garlic cloves, and jalapeño on the hot grill. Turn frequently for 7-10 minutes until charred and softened.
Remove the ingredients from the grill and allow to cool before making your salsa.
Add the roasted onion and garlic to the molcajete first, and mash until smooth.
Next, remove the stem and deseed the jalapeño. Feel free to leave seeds if you like it on the spicy side. Add the jalapeño and roasted tomatoes and mash to your preferred texture. You can make it however chunky or smooth you like it. This process can also be done by pulsing in a blender.
After mashing, season to taste with salt and pepper, and stir in the chopped cilantro.
This salsa is best served after it sits for a while so the flavors can marry, but you can enjoy it right away if you want!