Preheat your deep fryer or large skillet with oil to 350°F (175°C). Spread frozen fries on a baking sheet and bake in the oven at 425°F (220°C) for 15-20 minutes until golden and crispy, turning halfway through. Remove and drain excess oil or transfer directly to the hot oil for an extra crispy finish.
In a heavy-bottomed saucepan, melt butter over medium heat until it begins to foam. Add flour and whisk continuously for about 1 minute until mixture turns light golden and smooths out, creating a roux.
Gradually pour in milk while whisking constantly to prevent lumps. Continue whisking until the mixture thickens, about 3-5 minutes, and begins to bubble gently.
Reduce heat to low and stir in shredded cheese gradually, allowing each addition to melt completely before adding more. Stir until the cheese sauce is smooth, glossy, and has a pourable consistency. Season with salt and hot sauce if desired.
Place the crispy fries on a serving plate or dish. Pour the hot cheese sauce generously over the fries, ensuring even coverage. The cheese should melt further with the heat of the fries, creating a gooey, stringy appearance.
Optionally, garnish with extra hot sauce or additional toppings. Serve immediately while hot, with cheese stretching as the fries are lifted.
