Butter a loaf pan then line with parchment paper or wax paper, one paper going along the width and one along the length (I cut mine to get a better fit) allowing a generous overhang of paper on all sides. Remove strawberry gelato from freezer and allow to soften about 10 minutes, or microwave several seconds till slightly soft but still holding its shape. Stir gelato and spoon mixture into loaf pan. Spoon in enough strawberry gelato to make a 1 inch layer, evenly smooth with a spatula. Cover with plastic wrap and freeze for 30 minutes.
Remove pistachio gelato from freezer, allow to soften about 10 minutes or microwave several seconds till slightly soft but still holding its shape. Stir pistachio gelato and remove loaf pan from freezer. Spoon in enough pistachio gelato (over strawberry layer) to make a 1 inch layer, evenly smooth with a spatula. Cover with plastic wrap and freeze for 30 minutes.
Remove chocolate gelato from freezer, allow to soften about 10 minutes or microwave several seconds till slightly soft but still holding its shape. Stir chocolate gelato and remove loaf pan from freezer. Spoon in enough chocolate gelato (over pistachio layer) to make a 1 inch layer, evenly smooth with a spatula. Cover with plastic wrap and freeze gelato bombe until completely hardened about 3 hours. To unmold bombe, remove from freezer and dip the pan in hot water or run hot water along edges being careful not to get water in pan. Lift bombe out using the overhang of parchement. Remove plastic wrap covering and invert onto a plate or cutting board. Run a sharp knife under hot water and slice spumoni gelato bombe into servings.
Strawberry Gelato
Whisk together sugar and cornstarch in a medium saucepan until blended. Stir in milk and cream. Cook over medium heat, stirring constantly, until gelato thickens and it begins to gently boil. Allow to gently boil about 30 seconds stirring constantly. Strain mixture into a bowl. Cover with plastic wrap pressing directly against the surface of the gelato to prevent a skin from forming. Refrigerate until completely cool, about 3 hours. Puree strawberries in in a food processor until completely pureed. Force pureed strawberries through a mesh strainer into the gelato and whisk. Stir in juice concentrate, if using. Process gelato in an ice cream maker according to manufactures directions. Transfer mixture to an airtight container. Freeze until firm.
Pistachio Gelato
Finely grind pistachios with 1/4 cup sugar in a food processor (being careful not to turn mixture into pistachio butter), set aside. In a medium saucepan whisk together cornstarch and 1/2 cup granulated sugar until well blended. Stir in milk and cream. Add finely ground pistachio mixture. Cook over medium heat, stirring constantly, until gelato thickens and begins to gently boil. Allow to gently boil about 30 seconds, stirring constantly. Remove from heat, stir in almond extract and green food coloring if using. Pour mixture into a bowl and cover with plastic wrap pressing directly against the surface of the gelato to prevent a skin from forming. Refrigerate until cool at least 3 hours. Once completely cooled, process gelato in an ice cream maker according to manufactures directions. Transfer mixture to an airtight container. Freeze until firm.
Chocolate Gelato
In a medium pan whisk together sugar and cornstarch until well blended. Stir in milk and cream, then add chocolate chips. Cook over medium heat, stirring constantly, until mixture begins to thicken and gently boil. Allow to gently boil about 30 seconds. Remove from heat. Pour mixture into a bowl, cover with plastic wrap pressing directly against the surface of the gelato to prevent a skin from forming. Chill at least 3 hours until completely cooled. Once cooled, process gelato in an ice cream maker according to manufacturers directions. Transfer gelato to an airtight container. Freeze until firm.