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2. **Make the pickling liquid:** In a small saucepan, combine the vinegar, water, sugar, salt, peppercorns, mustard seeds, garlic, and red pepper flakes (if using). Bring to a simmer over medium heat, stirring until the sugar and salt dissolve.
3. **Pour the brine:** Carefully pour the hot brine over the tomatoes, onions, and cucumbers in the jar(s), making sure all the vegetables are fully submerged.
5. **Seal and cool:** Let the jars cool to room temperature, then cover tightly with lids and refrigerate.
6. **Pickling time:** Allow the veggies to pickle for at least 24 hours before serving for best flavor. They can last up to 2 weeks in the fridge.
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