Place the whole tomatoes and the jalapeño pepper in a 4-quart sauce pot with 2 quarts water. Bring the water to a boil and simmer the tomatoes and pepper for 10 minutes. Remove the tomatoes and jalapeño from the pot and chill in a bowl of ice water for 2 or 3 minutes.

Peel the tomatoes using a sharp knife. The skin should fall off easily. Cut the top off the jalapeño and discard. Remove the pepper's seeds because they contain most of the heat.

Place the tomatoes and the jalapeño in the bowl of a food processor. Add the garlic, the white onion and the fresh cilantro and purée until the mixture is smooth. Season the salsa with kosher salt to taste and pour into a bowl.