Remove any outer leaves from the cabbage. Cut the cabbage in half through the core. Slice one half finely into thin shreds using a sharp knife or mandoline, then place the shredded cabbage in a large mixing bowl. Sprinkle with the kosher salt and toss well to combine. Let sit for 20 minutes to soften slightly.

Meanwhile, in a medium saucepan, combine the apple cider vinegar, sugar, water, black peppercorns, mustard seeds, garlic, and bay leaf. Bring to a simmer over medium heat, stirring occasionally to dissolve the sugar. Once simmering, remove from the heat.

Transfer the cabbage to a clean mason jar or glass container, packing it down slightly. Pour the hot brine over the cabbage, making sure all the cabbage is submerged. Discard bay leaf if desired, or keep it for added aroma.

Let the mixture cool at room temperature for 20 to 30 minutes, then cover and refrigerate. The pickled cabbage is ready to eat after 1 hour but tastes even better after 24 hours. It will keep in the refrigerator for up to 3 weeks.