Heat a drizzle of oil in a small pot set over medium-high heat and saute the onions for 4-5 minutes, until soft but not browned. Add the garlic and cook for another minute. Add the tomatoes, brown sugar, vinegar, Sriracha and salt and bring to a simmer.
Cook, stirring when you think of it, for 45 minutes to an hour, until it thickens and looks like chunky ketchup. Remove from the heat and puree with a hand-held immersion blender right in the pot, or let cool and pour into a blender to puree until smooth. Transfer to jars and store in the fridge for up to a month, or freeze.