Thinly slice the cucumbers, onion, and bell peppers.
Pack the vegetables tightly into a clean glass jar or container.
In a bowl or measuring cup, whisk together vinegar, water, salt, sweetener (if using), peppercorns, and red pepper flakes.
Pour the pickling liquid over the vegetables until fully submerged.
Seal the jar and refrigerate for at least 2 hours (overnight is best).
Stir or shake before serving to redistribute flavors.
