Slice the sweet bell peppers and red onion into thin strips.

In a saucepan, combine white vinegar, water, sugar, salt, black peppercorns, mustard seeds, and red pepper flakes.

Bring the mixture to a boil over medium heat, stirring until the sugar and salt dissolve.

Place the sliced vegetables into a clean Mason jar.

Pour the hot pickling liquid over the vegetables, ensuring they are fully submerged.

Seal the jar and let it cool to room temperature before refrigerating.

Allow the pickles to marinate in the refrigerator for at least 4 hours before serving for best flavor.