Hull the strawberries. Peel and seed watermelon, cut into thumb sized chunks.
Put watermelon and sugar into a blender and blend until smooth. Pour mixture into chilled bowl.
Juice the lemon. Blend strawberries in the blender and slowly add yogurt and lemon juice while blending. Pour into bowl with watermelon and stir well. Put into the fridge for at least 2 hours. Use the fresh mint to garnish.