Melt 1/4 cup of butter in a saucepan over medium heat. Stir in the chopped onion, bell pepper, green onions, and garlic. Cook and stir until the onion has softened and turned translucent, about 5 minutes. Pour in the evaporated milk and bring to a simmer. Stir in the processed cheese until melted. Reduce the heat to low, and keep the sauce warm.

Meanwhile, melt 1/2 teaspoon of butter in a large skillet over medium heat. Add the crawfish tails and season with salt, pepper, garlic powder, and Cajun seasoning. Cook and stir until the tails are hot and beginning to curl, 3 to 5 minutes. Stir the crawfish tails into the cheese sauce, and cook for 5 minutes.

Heat oil in a large skillet to 350 degrees F (175 degrees C).

Cook the pistolette rolls in the hot oil until golden brown on all sides. Remove to a paper towel lined plate to cool. Once cool enough to handle, cut a slit in one end of each roll, then use the handle of a spoon to create a pocket in the roll. Stuff the rolls with the crawfish mixture and serve immediately.