Preheat the broiler to 500 degrees F.
Line a baking sheet with parchment paper and place the tomatoes and jalapeno pepper along 1 end of the baking sheet. Place the onion and garlic along the other side of the baking sheet and broil close to the heat source until the onions and garlic are blistered and beginning to char, about 10 minutes. Remove from the oven and, using tongs, transfer the onions and garlic to a small bowl to cool. Return the tomatoes and pepper to the oven and continue to broil until the tomatoes are charred around the edges and very soft and bubbly, about 10 more minutes. Remove from the oven and allow to cool.
When cooled, slip the garlic cloves out of their skins and transfer to the bowl of a food processor along with the onions, tomatoes, jalapeno pepper and any accumulated juices from the baking sheet. Process in short pulses until salsa is thoroughly combined but still slightly chunky. Add salt, cilantro, lime juice, sherry vinegar and water and stir to combine. Transfer to a bowl and set aside until ready to assemble the tacos.
Yield: about 1 1/2 cups salsa