Drain the canned tomatoes in a colander placed over a large bowl, allowing the juice to collect in the bowl below. Set aside.
Add the quartered onion, the smashed clove of garlic, the jalapeño, the cilantro (1/2 cup), the lime juice and zest, the kosher salt (1/2 teaspoon), and the sugar, if using (1/4 teaspoon) to a food processor. Press on for about 10 seconds, or until all the ingredients are very finely chopped.
Add tomatoes, reserving juice.
Pulse until 5-10 times, until your desired consistency (I like a few chunks, but not too many!). If it's thick, you can a little bit of the reserved tomato juice until the desired thickness (use any remainder for another purpose or discard).
For the best flavor, allow to refrigerate in an airtight container, jar, or covered bowl for a couple of hours or overnight. Serve with tacos, tortilla chips, or in your favorite recipe that requires salsa.
For a spicy salsa, leave all or some of the seeds and ribs in the jalapeño. For a mild salsa, remove the seeds and ribs. I use a spoon to scoop them out of a halved pepper, being careful not to touch them with my hands (you can wear gloves to be extra cautious if you like).
Make ahead: This salsa tastes best when chilled overnight in the fridge, allowing the flavors to marry, before serving. Store in a jar or airtight container, and use within a week.