Russet potatoes work nicely for this delicately flavored fix-ahead potato dish. The pimientos add a good dash of color; the potatoes cook up tender but not mushy.
Cook the potatoes and onion in salted water for 5 minutes. Drain. Pour into a buttered 2-quart casserole dish. Add pimientos. Add the cream cheese to the white sauce and pour over the potatoes. Bake in a moderate (350 degrees F) oven for 45 minutes.