Prepare Vegetables: In a large mixing bowl, combine the sliced red and diced green bell peppers, chopped green tomatoes, and finely minced yellow onions. Mix them thoroughly and set aside.
Prepare Brine Mixture: In a saucepan over medium heat, combine 1 cup of aromatic cider vinegar, kosher salt, granulated sugar, brown mustard seeds, spicy chili flakes, warm sweet paprika, and smoky smoked paprika. Stir the mixture and bring it to a gentle boil, ensuring that the sugar and salt dissolve completely.
Cooking Process: Once the mixture reaches a gentle boil and the sugar and salt have dissolved, reduce the heat and let it simmer for about 5 minutes, allowing the flavors to meld together.
Combine the Ingredients: Pour the simmered brine mixture over the prepared vegetables in the mixing bowl. Stir everything thoroughly to ensure the vegetables are well coated with the brine.
Marinate and Store: Let the mixture cool to room temperature. Once cooled, transfer the Piccalilli into sterilized jars, ensuring they are airtight. Pour the remaining 1/2 cup of aromatic cider vinegar evenly among the jars to cover the Piccalilli.
Seal and Refrigerate: Seal the jars tightly and store them in the refrigerator. Allow the Piccalilli to marinate and develop its flavors for at least a week before consuming.
Enjoy: Piccalilli tastes best after the flavors have had time to meld. Serve it as a delightful accompaniment to various dishes or enjoy it with sandwiches, meats, or cheeses.