Bake the waffle fries as the label directs. Meanwhile, cook the bacon in a large skillet over medium heat until crisp, about 4 minutes per side. Drain on paper towels, then chop into bite-size pieces.
Combine the cheddar, mozzarella, cornstarch, pimientos and mustard powder in a food processor and pulse until smooth. Transfer to a medium saucepan; add 1/3 cup water and cook over medium heat, stirring constantly, until the cheese melts and the mixture is smooth, about 5 minutes. Stir in the sour cream and season with salt and pepper.
Divide the waffle fries among plates; top with the cheese sauce, bacon and scallions.