Cook the potatoes: Wash and scrub the potatoes. Place them in a large pot and cover with water. Bring to a boil and simmer until just fork-tender, about 15-20 minutes. Do not overcook them to maintain texture.
Drain and cool: Drain the potatoes and let them cool slightly. If desired, cut larger potatoes in half or quarters to ensure uniform size.
Prepare the brine: In a separate saucepan, combine the vinegar, water, olive oil, salt, peppercorns, bay leaves, and chili flakes (if using). Bring the mixture to a boil, then reduce heat and simmer for about 5 minutes to infuse the flavors.
Assemble in jars: In a clean glass jar or large container, layer the cooled potatoes, sliced onion, chopped garlic, and diced bell pepper (if using).
Marinate: Pour the hot or cooled brine mixture over the potatoes, ensuring they are completely submerged. Add the fresh parsley.
Seal and chill: Seal the jar tightly with a lid. Allow the mixture to cool to room temperature, then place it in the refrigerator.
Serve: The potatoes will be best after marinating for at least 24 hours, and ideally 48 hours, to allow the flavors to meld. They will keep well in the refrigerator for up to 2 weeks.