place tomatoes, habanero pepper, garlic clove, and chopped onion on a baking sheet
roast in the oven for about 20 min or until the tomatoes are blackened.(Check after about 15 min. If the onions and garlic are toasted you can go ahead and remove those return the tomatoes and habanero to roast some more until roasted thoroughly. The skins should be blackened.)
In the molcajete grind the onion down until juices start to release, then grind the garlic, habanero and tomatoes.