Hull and quarter the strawberries into 1/2-inch chunks, then set aside in a large bowl. Dice the onions and red onions into small, uniform pieces. Finely dice the jalapeños into 1/8-inch pieces, removing seeds if you prefer less heat. Dice the red and green bell peppers into similar-sized pieces. Measure out the cilantro and mint, chopping them roughly. Juice the limes fresh to capture maximum brightness. Having everything prepped and ready before you start cooking is essential for canning—it keeps the salsa at the right temperature during assembly and ensures consistent quality.

In a large, heavy-bottomed pot (at least 8-quart capacity for proper boiling and canning), combine the cider vinegar, sugar, and agave over medium heat. Stir frequently until the sugar and agave are completely dissolved—this typically takes 3-5 minutes. The vinegar's acidity is critical for safe canning, so using vinegar with at least 5% acidity ensures proper preservation. Once fully dissolved, increase the heat to medium-high to prepare for the next phase.

Add the diced onions, red onions, jalapeños, and bell peppers to the hot vinegar mixture. Stir well to combine. Add the cilantro, mint, lime juice, canning salt, and red pepper flakes if using. Bring the mixture to a rolling boil over medium-high heat, stirring occasionally to ensure even cooking and prevent sticking. Let it boil for exactly 5 minutes—this short boil develops the flavors while the vinegar begins its preserving work. I find that boiling for this precise timing creates a bright, balanced salsa rather than one that tastes stewed or overly cooked.

Remove the pot from heat immediately after the 5-minute boil. Gently fold in the quartered strawberries using a wooden spoon or spatula, stirring just enough to distribute them evenly throughout the hot mixture. Stir in the Ball Pickle Crisp or calcium chloride, which helps the berries maintain their firm texture and bright appearance during the water bath processing. Let the salsa cool slightly for 2-3 minutes before jarring—the residual heat will help seal the jars properly, but the salsa shouldn't be at a rolling boil when it goes into jars.

Ladle the hot salsa into sterilized canning jars, leaving exactly 1/2-inch headspace at the top—this space is essential for proper sealing and safe preservation. Wipe the jar rims clean with a damp cloth to remove any drips or residue that could prevent a proper seal. Place the hot lids and rings on the jars, tightening the rings just until snug (don't over-tighten). Carefully place the filled jars on the rack in a boiling water bath canner, ensuring they're covered by at least 1-2 inches of water. Process for 20 minutes at a gentle rolling boil. I always start my timer after the water returns to a full boil, not when I place the jars in, to ensure proper processing time.

Remove the jars from the boiling water bath using a canning jar lifter and place them on a clean kitchen towel or wooden cutting board, spacing them apart to cool. Let them cool completely at room temperature for 12-24 hours without moving them. You'll hear a satisfying 'pop' sound as each jar cools and the lid seals—this indicates successful preservation. Check that all lids have sealed by pressing the center of each lid; it should be slightly concave and not flex when pressed. Any jars that didn't seal should be refrigerated and used within a few weeks.