Make the chimichurri first. Combine the parsley, garlic, oregano, chili flakes, and red wine vinegar in a bowl. Whisk in the olive oil, season with salt and pepper, and set aside at room temperature.
Pat the sardines dry with paper towels. If using fresh sardines, season lightly with salt.
Stir together the fish sauce, rice vinegar, and water in a small bowl. Set next to the stove.
Heat the sugar in a dry, wide skillet over medium heat. Swirl gently once the edges liquefy. Continue until the sugar turns deep amber, about 3 to 4 minutes.
Immediately pour in the fish sauce mixture; it will bubble and seize briefly. Stir until smooth and glossy, about 1 minute.
Add the garlic and ginger and simmer 2 minutes. Add the sardines, spooning sauce over them, and cook 3 to 4 minutes per side (or 2 minutes total for tinned sardines) until glazed.
Transfer the sardines to a plate, spoon over remaining glaze, and top with chimichurri.