Heat the oven to 375 degrees F. Butter a large oval casserole or souffle dish, set aside.
Using a large heavy-bottomed saucepan over high heat, bring water, milk, and 2 teaspoons of salt to a boil. Gradually stir in the grits; reduce the heat to medium, and cook, stirring often, until thick and creamy, approximately 30 to 45 minutes.
Using a large bowl, whisk the egg yolks, temper by whisking in a spoonful of hot grits. Stir yolks into remaining grits. Add the cheese, garlic, and butter. Set aside to cool slightly.
In the bowl of a heavy-duty mixer fitted with the whisk attachment, whisk the egg whites with a pinch of salt on medium speed until foamy. Increase the speed to high and whip until stiff peaks form, approximately 2 to 3 minutes; set aside.
Add about a quarter of the beaten egg whites to the grits mixture and stir until well mixed. Pour this mixture and the scallions over the remaining whites and fold them together as lightly as possible. Pour the mixture into the prepared dish.
Bake until the grits are set, approximately 30 to 40 minutes. (If the surface begins to over brown, cover with foil.) Serve immediately.
Parmigiano Reggiano Grits Souffle Recipe: https://whatscookingamerica.net/pasta_rice_main/parmigianogritssouffle.htm