Cook potatoes in large pan with water salted with 1 Tablespoon of Kosher salt just until tender, 4 to 5 minutes after the water comes to a boil. Drain in colander.
Preheat oven to 375° F.
In a large casserole dish, layer the ingredients as follows: 1/3 of the drained potatoes, 1/3 of the flour (about 1½ Tablespoons), 1/3 of the butter (about 1½ Tablespoons), freshly ground black pepper, and 1/3 of the sliced Velveeta cheese (about 2½-ounces). Repeat for 2 additional layers. Pour milk evenly over the top.
Bake in a preheated oven for 50 minutes, or until top is browned and mixture is bubbly. Remove from oven and top with shredded Cheddar cheese. Return to oven and bake for 10 more minutes, or until cheese is melted and bubbly. Remove from oven and serve. Yield: 6 to 8 servings.