Peel the red onions and trim off the root and tip. Slice the onions crosswise into thin half-moons, about 1/8-inch (3 mm) thick. You should have roughly 2 to 2 1/2 cups of sliced onion. Place the sliced onions in a clean heatproof jar or bowl that will hold about 2 cups.

Make pickling liquid

In a small saucepan combine the white vinegar, apple cider vinegar, water, granulated sugar, and kosher salt. Add the black peppercorns, coriander seeds, bay leaf, and red pepper flakes. Bring the mixture to a low simmer over medium heat, stirring to dissolve the sugar and salt. Once it simmers for 1 minute and the solids are dissolved, remove from heat.

Combine and steep

Carefully pour the hot pickling liquid over the sliced onions in the jar so the onions are fully submerged. Use a spoon or tongs to press the onions down if needed. Let the jar sit uncovered at room temperature for 30 minutes to cool slightly, then cover with a lid or plastic wrap.

Chill and develop flavor

After the jar reaches room temperature, refrigerate for at least 1 hour before using for a bright, mild pickle. For best flavor, refrigerate 8–24 hours. Pickled red onions will keep in the refrigerator for up to 3 weeks. Discard if cloudy or off-smelling.

Optional Garnish / Serving

When ready to serve, drain a few tablespoons of pickling liquid if desired and toss the onions with fresh cilantro leaves or serve with lime wedges alongside tacos, salads, sandwiches, grilled meats, or bowls.