Half-fill a large saucepan with water and bring to a boil over high heat. Add the carrots, reduce the heat, and simmer, uncovered, for a few more minutes; then add the mushrooms and squash. Simmer, uncovered, for 3 minutes more or until almost tender.

Drain the vegetables thoroughly in a colander, rinse with cold water, and drain again.

Transfer the vegetables to a large glass bowl. Stir in the cucumber pieces and red onion slices. Toss to combine.

Pour on the white wine vinegar, water, and olive oil. Add the marjoram, crushed black peppercorns, coriander, mustard seeds, and salt. Cover to toss and blend.

Refrigerate at least 6 hours prior to serving. Make sure to stir or toss occasionally to balance the savory flavors throughout the vegetables.

To serve, use a slotted spoon to remove vegetables from marinade. Serve with picks, if desired.