Many mexican recipes call for tomatoes to be asados (roasted).
Traditionally they are put onto a hot comal and cooked until the skin is wrinkled and brown and the flesh is soft right through--this takes about 20 to 25 minutes for an 8 ounce tomato.
However, since this method is very messy, it is best to line a shallow metal pan with foil and put the tomatoes in it.
Place them under a hot broiler--do not have the flame too high or the tomato will burn without cooking through--and turn them from time to time so that they cook through evenly--the skin will be blistered and charred.
A medium tomato will take about 20 minutes.
Blend the tomato, skin, core, and seeds to a fairly smoothe sauce.
The skin and core give both body and flavor to the sauce.
And never mind if the skin ~is- charred; that adds character, too.
If the skin is very badly blackened and hard in places, then remove a little of it.
This method of cooking tomatoes makes for a very rich-flavored sauce