Prepare the sliced potatoes, and cover them with cold water in a large container, and set aside. In a medium saucepan, melt 3 tablespoons of butter or margarine. Remove from heat and stir in 1/4 cup flour, stirring until lumps are dissolved. Add 1 teaspoon salt, 2 cups milk, and 2 cups shredded Velveeta cheese. Return mixture to low heat, and cook, stirring constantly, until cheese sauce is thick and bubbly. Remove from heat. Stir in 1 tablespoon fresh chopped parsley. Spray a very large casserole dish (13-inch by 9-inch by 2-inch) with cooking oil spray, and place 1/3 of the cheese sauce on the bottom of the dish. Spread it out evenly. Now, place 1/2 of the sliced potatoes in a layer on top of the cheese sauce. Sprinkle 1/4 cup finely chopped onion over the potatoes, salt and pepper to taste, and pour another third of the cheese sauce over the top. Next, place the other half of the sliced potatoes in a even layer over the top of the casserole. Salt and pepper to taste, and pour the final third of the cheese sauce evenly over the entire casserole. At this point, if your cheese sauce does not come up to the level of the potatoes, add milk, distributing it evenly, until there is liquid all the way to the top of the potatoes. (I used 2 additional cups of milk.) Cover casserole with aluminum foil, and bake at 350 degrees for 30 minutes. Remove the foil, and bake 45 minutes longer. Test the potatoes with a fork, and, if they are soft, remove the casserole from the oven. Place the foil back on top of the casserole, and let it sit on the counter for about 10 minutes to let the potatoes soak up all the liquid. Remove the foil, and your Scalloped Potatoes with Cheese is ready to serve! It will look gorgeous, and it is very tasty! You'll love it!!