Blanch the beefsteak tomatoes in boiling water for 1-2 minutes, then transfer to ice water. Peel off the skin, cut in half, and remove the stalk base and seeds.
In a saucepan, combine 50 ml water and 50 grams sugar. Bring to a simmer over medium heat and simmer for 10 minutes, stirring occasionally. Remove from heat and let cool to room temperature.
Remove the seeds from the watermelon pulp. Cut the red chilli pepper in half lengthwise, remove the seeds, and cut the flesh into small pieces. Peel the ginger and chop into small pieces.
Combine the prepared tomatoes, watermelon pulp, cooled sugar syrup, chilli pepper pieces, and ginger in a food processor or blender. Puree until smooth.
Pour the mixture into a shallow bowl and place in the freezer. Every 30 minutes for 2-3 hours, stir the mixture with a fork to break up ice crystals.
Transfer the granita to chilled glasses and serve, optionally garnished with fresh mint.