1. To prepare the pickle, combine the vinegar, 3 cups of water, sugar, coriander seeds, mustard seeds, shallot, garlic, and cilantro in a small saucepan. Boil the liquid over medium-high heat, reduce to simmer, and cook for 5 minutes. Switch off the flame and let it cool completely.
2. Place the feta in a large mixing bowl or divide it between two jars. Pour the cool pickling liquid, we prepared earlier over the cheese. Feta should be completely submerged. Cover the feta and refrigerate it for at least five days.
3. To prepare the salad, remove the pickled feta from the liquid and drain it on paper towels. Take a bowl and combine the olives, strawberries, olive oil, cilantro, and spinach. Add feta, then season with salt and pepper.