Combine water and cranberries in a small saucepan and bring to a boil. Reduce heat and simmer until cranberries are soft and liquid is syrupy, 5 to 10 minutes. Stir in honey, remove from the heat, and allow to cool.

Put kale and red onion in an 8-quart pot or a very large bowl.

Whisk olive oil, balsamic vinegar, garlic, salt, and pepper together in a small bowl; pour over the kale and stir well. Add feta cheese, walnuts, and moist cranberries with their syrup; stir until well combined. Cover and place in the refrigerator to marinate, stirring occasionally, until kale has wilted and shrunken in size, 3 to 8 hours.

To serve, transfer to a smaller serving bowl.