Prep the vegetables: Core the cabbage and finely chop it. Chop tomatoes, onions, bell peppers, celery, and jalapeños (if using) into small, even pieces. Aim for a small dice so the relish spreads easily.

Salt and drain: In a large bowl, toss the chopped vegetables with 2 tablespoons kosher salt. Let sit for 1 hour to draw out excess moisture. Stir once or twice during that time.

Rinse and squeeze: Transfer the salted vegetables to a colander. Rinse under cool water to remove excess salt, then squeeze by the handful to remove as much liquid as you can. This step keeps the relish crisp and prevents a watery brine.

Make the brine: In a large nonreactive pot, combine apple cider vinegar, white vinegar, sugar, mustard seeds, celery seeds, turmeric, ground mustard, red pepper flakes (if using), 1 teaspoon salt, and 1/2 cup water. Bring to a simmer over medium heat, stirring until the sugar dissolves.

Add the vegetables: Stir in the drained vegetable mixture and the minced garlic. Return to a gentle simmer and cook for 8–10 minutes, stirring occasionally. You want the flavors to mingle while keeping the vegetables slightly crisp.

Taste and adjust: Taste the brine. Add more sugar for sweetness, more vinegar for tang, or a pinch more salt if needed. The flavor should be bold, bright, and balanced.

Cool and pack: Remove from heat and let the relish cool for 15–20 minutes. Ladle into clean jars, making sure the vegetables are submerged in brine. Leave about 1/2 inch of headspace at the top.

Chill and rest: Seal and refrigerate for at least 24 hours before serving. The relish tastes best after 2–3 days as the flavors meld.