Salt the veggies: In a large bowl, toss sliced cucumbers, onions, and bell pepper (if using) with the salt. Cover with plastic wrap and let sit in the fridge for 2 hours. This step draws out excess water and helps keep the pickles crisp. Rinse well under cold running water, then let drain thoroughly.
Make the brine: In a medium saucepan, combine the vinegar, granulated sugar, brown sugar, garlic, mustard seeds, celery seeds, turmeric, cloves, and red pepper flakes. Bring to a simmer, stirring until the sugar dissolves, a few minutes. Remove from the heat and cool to room temperature (you can speed this up by transferring the brine to a heatproof container and chilling in the fridge).
Pack the jars: Divide the cucumbers, onions, and peppers between 4 clean one-pint jars (or two 1-quart jars). Pour the brine over top, making sure the pickles are fully submerged. (If you come up a little short on brine, it's fine to top off the jars with a splash of water—just make sure the cucumbers are fully submerged.)
Pickle: Seal with lids and refrigerate for at least 24 hours. They will keep nicely for up to 1 month in the fridge.
