Put the chunked Tomato into a blender and purée until completely smooth. Set a fine sieve over a medium bowl, pour the Tomato purée into the sieve, and allow to drain for at least 10 minutes. Reserve the resulting Tomato water, which should be about 1/4-1/3 cup.
In another medium bowl, combine the chopped Mint, Watermelon, Tomato cubes, salt, a few grinds of pepper, the olive oil, and the champagne vinegar. Toss gently to combine.
Prepare salads individually by spooning a tablespoon of the reserved Tomato water onto a plate (chilled if you like), topping it with a quarter of the Watermelon and Tomato mixture, sprinkling 2 tablespoons of crumbled feta on top, and garnishing with a Mint leaf. Serve immediately. Serves 4.: