Heat the milk and heavy whipping cream in a saucepan over medium-high heat until it begins to simmer and foam.
Meanwhile, in a mixing bowl, whisk together the egg yolks and sugar using a whisk or hand mixer. Continue whisking/mixing until the mixture is smooth and ribbon-like.
Once the milk begins to simmer, remove it from the heat. Slowly pour half of it into the egg mixture while whisking rapidly. Working quickly, transfer the egg mixture into the saucepan, pouring in a slow stream and whisking rapidly until everything is combined. It is important to work quickly here so that the egg doesn't cook and create little chunks of cooked egg.
Heat over medium heat and whisk in the pistachio nut paste. Continue heating until the mixture starts to thicken.
Remove from heat and pour the mixture into a sturdy, freezer-safe pan, like a bread pan. Place in freezer for 30 minutes.
Because we are doing this by hand rather than an ice cream maker, it is important to churn the gelato by hand as it freezes in order to break up the ice crystals and create a smooth texture. Whisk (or use a hand mixer) on the gelato every 30-45 minutes until it reaches the texture of frozen yogurt, then simply leave it in the freezer until ready to eat. This should take 3 to 4 hours, or about 6 "churnings."
Let sit for 5-10 minutes on the counter to soften before serving. Sprinkle pistachio nuts over the gelato if desired.