Place potatoes in large saucepan and cover with cold water. Over medium high heat, bring to boil and cook about 30 minutes until very tender. Meanwhile, place tablespoon butter and sliced onion in large skillet and allow to saute over medium heat. Cook for 5 to 8 minutes until tender. Remove from heat and set aside.
Preheat oven to 350 degrees. Cook pasta according to package directions. Drain.
Drain potatoes when tender and add butter, milk, and salt and mash until smooth. Add cubed Velveeta and mix until cheese melts.
Butter a casserole dish and layer half the cooked, drained pasta in bottom of dish. Cover pasta with half the potato mixture and half the sauteed onions. Repeat layers ending with the onions. Bake in preheated oven for 30 minutes or until heated thoroughly. Remove from oven and let stand for 5 minutes, then serve. Makes 6 to 8 servings. Enjoy!