Preheat the broiler. In a medium pan, bring the milk and 2 cups water to a boil. Whisk in the grits and cook, stirring, until thickened, about 5 minutes. Add the butter; stir until melted. Stir in half the cheese and half the pimientos.
Transfer the grits to a fondue pot or other ovenproof pot; top with the remaining cheese and pimientos. Broil until the cheese browns, 3 to 5 minutes. Rim the pot with the shrimp; garnish with the scallions.