In a bowl, combine 1 cup of sugar and 2 tbsp of corn starch. Prepare 2 cups of whole milk, and use a small part of the milk to dissolve sugar and corn starch. Whisk it well, and make sure there are no lumps.
Heat up a pot over medium heat, add all of the liquid mentioned in step 1 into the pot, together with 1 cup of heavy cream. Stir well. Heat it up until about to boil (when there are small bubbles appearing at the edge of the pot), turn off the heat, and let it simmer for 8 minutes.
In a clean bowl, whisk one egg yolk. Add a spoonful of hot milk mixture from the pot to the yolk. Repeat this step 3 times to gradually increase the yolk’s temperature (otherwise, the yolk will become scramble eggs). Make sure there are no lumps in the yolk, and then pour it into the pot, mix well.
Remove the milk from the heat, and wait until the mixture completely cools down. You can use an ice bath to speed up the process, it will take about 3 hours.
Prepare 2 cups of fruit juice/puree. For more detail please check the video below, we were using watermelon and mango in the video.
Add the juice into the cold milk mixture, mix well. Transfer it into a container, and freeze it.
While freezing the gelato, the icy part will be formed. We will need to take it out and churn the gelato mixture every 20-40 minutes. You can whisk until all of the icy parts disappear. Repeat the process at least 6 times until the texture becomes creamy. After the last time of churning, you can leave the gelato in the freezer overnight and enjoy it the next day.
Before serving it, take the gelato out of the freezer and leave it at the room temperature for 15 minutes. Enjoy!