Heat oven to 375 degrees.
Coat a 2-quart baking dish with cooking spray.
Melt butter in a large frypan over medium heat; add in onion and cook stirring occasionally until onion begins to brown (about 5 minutes).
Stir in flour, then slowly add in milk.
Add in potatoes; stir to mix; bring to a boil.
Stir in 3/4 cup cheese, salt and black pepper.
Pour mixture into prepared baking dish then spread out.
Bake for 1 hour uncovered.
After 1 hour cooking time cover with foil and continue to bake another 20 minutes more.
Change oven to broil heat.
Sprinkle the remaining cheese over potatoes and broil 6-inches from heat until the cheese is golden (about 1-2 minutes).
Cool 5 minutes before serving.
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