Place a freezer safe container in the freezer to chill.

In a medium saucepan whisk together the cream, milk, sugar, cornstarch and salt, then place over medium heat and cook, stirring constantly, until the mixture comes to a boil.

Let boil for one minute, whisking the whole time, to activate the cornstarch.

Remove from the heat and stir in the watermelon puree and lemon juice.

Pass the mixture through a fine mesh strainer to remove any pulp or fibers.

Refrigerate the mixture until fully cold, at least 2 hours.

Churn according to the directions for your ice cream maker. I suggest churning until it resembles the consistency of soft serve.

Transfer the gelato to the chilled container, cover and freeze for a minimum of 4 hours. The remaining gelato can be kept in the freezer for up to 4 weeks.