Heat oven to 375.

Coat a 2-quart baking dish with Pam.

Melt butter in a large pan over medium heat. Add onion and cook, stirring occasionally, until onion begins to brown (about 5 minutes). Stir in flour. Add milk slowly, stirring. Add potatoes, stir to mix. Bring to a boil. Stir in 3/4 cup of the cheese, salt, and pepper.

Pour mixture into the baking dish and spread out. Bake for 1 hour uncovered. Cover and bake until potatoes are fork tender, about 20 minutes more.

Change oven temperature to broil. Sprinkle remaining cheese over potatoes and broil 6” from the heating element until the cheese is golden brown (about 1-2 minutes).

Allow to cool for 5 minutes before serving.