Preheat oven to 350 degrees. Spray 9x13 pan with nostick spray. Peel and quarter 10 medium potatoes and place in salted water. Bring to a boil and cook until fork tender. Drain off water. Place a stick of butter in pot with potatoes and let melt. Season potatoes with salt and pepper to taste. Beat potato mixture by hand or with mixer. Add 1/3 cup milk while beating. Depending on texture of potatoes, it might take a little more or less milk to get a creamy (but not runny) mixture. Set aside.
Put meat in large skillet on medium high. Season with a little salt and pepper to taste. Chop large onion and add to meat while browning. When meat and onion are cooked well, add taco seasoning mix and the amount of water it calls for (the Old El Paso called for 1/3 cup). Turn meat mixture down and simmer for about 10 minutes. Add corn and salsa and simmer for about 5 more minutes to blend flavors.
To begin assembling, place meat mixture evenly in the bottom of the 9x13 pan. Next, sprinkle just a little of the grated cheese evenly over the meat mixture. Next, spread the mashed potatoes evenly over the meat mixture. Then, sprinkle the bulk of the grated cheese over the potatoes. Place in oven and bake 35 to 40 minutes or until cheese is fully melted and dish is starting to turn golden brown and bubbly.