Preheat your oven to the temperature indicated on the package of your frozen French fries. Spread the fries evenly on a baking sheet in a single layer. Bake according to package instructions, usually around 20-25 minutes, flipping halfway through until they are crispy and golden brown.
While the fries are baking, prepare the cheese sauce. In a medium saucepan, melt the unsalted butter over medium heat. Once melted, add the all-purpose flour and whisk continuously for about one minute to create a roux.
Gradually pour in the whole milk, continuing to whisk until the mixture is smooth and begins to thicken. This should take about 2-3 minutes.
Reduce the heat to low and add the shredded cheddar cheese to the saucepan. Stir until the cheese is completely melted and the sauce is creamy. If the sauce is too thick, you can add a bit more milk to reach your desired consistency.
Once the fries are ready, remove them from the oven and transfer them to a serving platter. Drizzle the warm cheese sauce generously over the fries. For an extra touch, consider adding toppings like chopped green onions, crispy bacon, or sliced jalapeƱos.