Preheat the oven to 400 degrees F. Generously butter a 9x9 or 8x8-inch casserole dish.
In a medium saucepan melt the butter over medium-low heat, whisk in the flour and cook 2 - 3 minutes while stirring.
Slowly whisk in the milk and enchilada sauce. Turn the heat up and bring to a simmer. Simmer 5 minutes, stirring often.
Remove from the heat and stir in 1 cup of the cheese, stir until melted.
Layer half the potatoes in the pan.
Pour half the sauce over them and sprinkle with half the remaining cheese.
Repeat the layers but sprinkle the top with corn chips then the cheese.
Bake 45 - 50 minutes or until the potatoes are fork tender. Cover with foil if the top starts to brown too much.
Garnish with green onions and a dollop of sour cream.