Prepare the Vegetables: Gather ingredients and remove the stems from the peperoncini. In a food processor, combine all peppers, onion, and garlic. Pulse until finely chopped for 1 to 2 minutes.

Cook the Mixture: Transfer chopped vegetables to a saucepan over medium heat. Stir in peperoncini juice, lemon juice, and red pepper flakes. Cook for 3 to 5 minutes until softened.

Drain and Cool: Carefully drain excess liquid. Spread mixture out and let cool completely at room temperature.

Final Touches: Stir in chopped parsley, olive oil, salt, and black pepper to taste until well combined.

Store and Serve: Transfer relish to an airtight container. Best enjoyed within 1 to 2 weeks. Use as topping on burgers, sandwiches, or salads.