Preheat oven to 400 degrees F (200 degrees C). Grease a 1-quart casserole dish.
Spread about 1/3 of the potatoes into the bottom of the prepared dish. Mix flour and salt together in a bowl; sprinkle about 1/3 of the mixture over the potatoes. Dot potato layer with about 1/3 of the butter. Sprinkle about 1/3 of the onion over the potatoes. Repeat layering twice more.
Stream milk over the potato mixture in the casserole. Sprinkle paprika over everything.
Bake in preheated oven until the top is nicely browned, about 1 hour.
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