Wash the tomatoes and make a shallow cross cut on top of each one, without cutting all the way through.
Combine the chopped parsley and garlic, then mix well.
Gently fill each tomato with the garlic and parsley mixture.
Place them tightly into a container or pot.
In a saucepan, combine the water, salt, sugar, bay leaves, peppercorns, allspice, and mustard seeds. Bring to a boil.
Remove from heat, add the vinegar, and let the marinade sit for 1–2 minutes.
Pour the hot (not boiling) marinade over the tomatoes, making sure they are fully covered. Place a small plate on top if needed to keep them submerged, then cover with a lid.
Let cool to room temperature, then refrigerate for 10–12 hours. They get even more flavorful the longer they sit.
