For the gelato: The night before, freeze the ice cream maker bowl, if needed.
Prepare a large bowl with ice and a little water.
Whisk together the sugar, salt and cornstarch in a medium saucepan. Put over medium heat and allow to toast until fragrant, 3 minutes, stirring constantly.
Slowly whisk in the milk and heavy cream. Whisking constantly, cook until the mixture begins to bubble and thicken, 5 to 6 minutes. Remove from the heat, pour into a medium heatproof bowl and place over the ice bath. Whisk constantly to help cool the mixture, 3 to 5 minutes.
Once cool, prepare an ice cream maker for freezing according to the manufacturer's instructions. Turn on the machine and slowly add the cooled gelato mixture. Allow to churn for 15 minutes until it thickens.
Meanwhile, make the wet walnut topping: Combine the corn syrup, maple syrup and lemon zest in a medium saucepan over medium heat. Bring to a boil and cook for 5 minutes. Remove from the heat and stir in the lemon juice, chopped walnuts and salt. Set aside.
When the gelato is thick and creamy, turn off the ice cream machine. Scoop the gelato into serving dishes, top with wet walnuts and any other toppings.
This gelato is best served straight from the machine. If you store it in an airtight container overnight, I like to remove it from the freezer 15 minutes before serving. Freeze for up to 1 month.