Rinse and slice jalapeños into 1/8–1/4 inch rounds. Remove seeds and membranes to reduce heat if desired. Wear gloves if you’re sensitive to capsaicin.
Combine vinegar, water, salt, sugar, garlic, peppercorns, coriander seeds, and bay leaf in a saucepan. Bring to a gentle simmer and stir until salt and sugar dissolve (2–3 minutes).
Pack jalapeño slices into two clean pint jars, adding onion or carrots if using. Leave 1/2 inch headspace.
Pour the hot brine over the jalapeños, ensuring they are fully submerged. Remove air bubbles, let jars cool to room temperature, then seal and refrigerate.
Allow at least 24 hours for flavor development; 2–3 days is ideal. Store refrigerated for up to 6–8 weeks.
Notes
Blanch slices for 30 seconds and shock in ice water to reduce heat quickly before pickling.
For sweeter pickles, increase sugar to 2 tablespoons and add a splash of apple cider vinegar.
If you prefer shelf-stable jars, use a tested water bath canning method.