1. **Prepare the vegetables:** Slice the cherry tomatoes in half, thinly slice the red onion, and slice the cucumber into thin rounds. Place them in a clean glass jar or several jars.

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.

4. **Add herbs:** If using fresh dill or basil, tuck some sprigs into the jars for extra flavor.

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.

### Notes:
– These pickled veggies are great on salads, sandwiches, or served as a snack or side dish.
– Feel free to adjust the seasoning (e.g., more garlic or extra herbs) based on your preference.

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