Cover whole potatoes with cold water by 1 inch in a saucepan. Bring to a simmer and cook for 12 to 15 minutes until the outer flesh yields to a knife but the centers remain firm; drain, cool for 10 minutes and refrigerate in a shallow dish for 45 minutes until easy to handle and firm enough to grate.
Heat the oven to 375°F. Peel cooled potatoes and grate on the large holes of a box grater, keeping the shreds loose. Grease a 2-quart baking dish with 1 teaspoon of the butter.
Melt remaining butter in a saucepan over medium heat and cook onion for 5 minutes. Whisk evaporated milk, broth and cornstarch together, pour into the pan and stir for 2 to 3 minutes until gently bubbling and slightly thickened.
Remove from the heat and stir in 6 ounces Colby, salt and pepper. Fold the sauce through the fresh potato shreds and spread in the prepared dish; scatter remaining Colby over the top.
Bake for 40 to 45 minutes until the center bubbles and a knife slides through the shreds without a firm catch. Cover loosely if the top browns before the potatoes soften, then rest for 10 minutes before serving.