How to Make It
Line the bottom and all the way up the sides of a 4-to 6-quart slow cooker insert with parchment paper. Trim any extra that might keep the lid from fitting snugly. Put the waffle fries or hash browns in the slow cooker and set aside.
Heat the oil in a large skillet over medium-high heat. Add the sausage and cook, breaking up with a flat-bottomed spatula and stirring occasionally, until golden brown and crispy, 6 to 8 minutes. Using a slotted spoon, transfer sausage to the slow cooker and let cool.
Meanwhile, whisk the eggs, milk, cheese, chives, 1½ teaspoons salt, and ½ teaspoon pepper in a large bowl. Pour the egg mixture over the waffle fries and sausage and smooth into an even layer.
Cover and cook on high for 2 to 2½ hours, or on low for 3 to 3½ hours, until the frittata is lightly golden brown and completely set.
Let cool 5 minutes, then grasp the sides of the parchment paper to lift the frittata out of the insert. Cut into slices and serve.