Heat oven to 350°.
In buttered 2 qt.casserole arrange ham; top with onion, then half of potatoes.
Sprinkle with half of flour and seasonings.
Meanwhile, heat milk with butter.
Pour over all.
Bake covered for 40 minutes; uncover and dot with catsup.
Then bake 30 minutes more or until potatoes are tender.
Serves 4 to 6.