Use a box grater to grate the cucumber into a clean kitchen towel; squeeze the water out and discard water (or use for a cocktail.) Add cucumber to a large bowl with yogurt, olive oil, garlic, lemon zest and juice, mint and feta. Whip or stir vigorously until fluffy. Stir in shallot, olives and tomatoes and season to taste with salt and pepper. For best results, chill at least one hour before serving.