Slice the cucumbers into 1/4-inch rounds and thinly slice the onion into half-moons. Smash the garlic cloves to release their oils. Divide the cucumbers, onion, and garlic evenly between two clean quart-sized jars (or similar size containers). To each jar, add half of the mustard seeds, peppercorns, fennel seeds, red pepper flakes, and celery seed. Tuck the fresh dill sprigs throughout the vegetables, distributing them evenly between both jars. I like to layer the dill between the cucumbers rather than just placing it on top—it infuses the brine more evenly.

In a medium saucepan, combine the water, cider vinegar, sugar, and salt over medium-high heat. Stir occasionally until the sugar and salt are completely dissolved, about 5-7 minutes. You'll know it's ready when there are no crystals visible at the bottom of the pan. Don't skip tasting the brine at this point—I always take a small spoonful to ensure the balance of sweet, salty, and tangy flavors is what I want before pouring.

Remove the brine from heat and let it cool for 3-5 minutes—this prevents shocking the vegetables with extreme heat while still being warm enough to properly infuse them. Carefully pour the warm brine into each jar, dividing it evenly and ensuring the cucumbers are completely submerged. Leave about 1/2 inch of headspace at the top of each jar. If any cucumbers float to the surface, gently press them down with a clean spoon or small weight to keep them submerged in the brine.

Allow the jars to cool completely to room temperature, about 1-2 hours. Once cooled, cover the jars and transfer them to the refrigerator. The pickles will start developing flavor immediately, but they'll taste best after 3-5 days when the flavors have fully melded. They'll keep refrigerated for up to 2 months.