In a small bowl, stir together 1/4 cup milk and the cornstarch until smooth to make a slurry; set aside.
In a 4-qt saucepan, whisk together the remaining milk, heavy cream, sugar, light corn syrup, and kosher salt.
Add the orange peel to the milk mixture and bring it to a boil over medium-high heat; cook for 4 minutes, stirring occasionally.
Stir the cornstarch slurry into the hot milk mixture, return to a boil, and cook, stirring, until thickened, about 2 minutes.
Place the mascarpone in a bowl and whisk in 1/4 cup of the hot milk mixture until smooth, then whisk this back into the remaining custard; stir in the puréed roasted beets until evenly combined.
Pour the mixture into a sealed plastic bag or container and submerge in a bowl of ice water to chill thoroughly, then remove and discard the orange peel.
Process the chilled mixture in an ice cream maker according to the manufacturer’s instructions, adding poppy seeds during the last minute of churning if using.
Transfer the churned ice cream to a storage container and freeze until firm before serving.