Preheat the oven to 400 to 425°F (200 to 220°C), following the package directions for your chosen brand of fries.

Line one or two baking sheets with foil and spray lightly with cooking spray to prevent sticking and increase crispiness. Spread the frozen fries in a single layer on the baking sheet(s). Do not overcrowd the fries, as this will cause them to steam rather than become crisp. If using two trays, divide the fries evenly between them.

Bake the fries according to the package directions, typically 22 to 30 minutes, flipping or shaking them halfway through for even cooking.

While the fries are baking, prepare the cheese sauce: Melt the butter in a medium saucepan over medium heat. Add the flour and cook for 1 to 2 minutes, whisking continuously until bubbly and slightly golden. Slowly add the milk, a little at a time, continuing to whisk to prevent lumps. Cook over medium-low heat, continuing to whisk until the mixture starts to thicken, about 3 to 5 minutes.

Turn the heat off and immediately remove the saucepan from the heat to prevent the cheese sauce from becoming grainy. Add the freshly shredded cheddar cheese and keep whisking until the cheese is completely melted and the sauce is smooth and creamy. Whisk in the salt. If the sauce appears too thick, you can whisk in an additional tablespoon or two of milk until your desired consistency is reached. Keep the sauce warm on the lowest heat setting until ready to serve.

When the fries have finished cooking, remove them from the oven and immediately sprinkle them generously with Cajun seasoning. Toss to coat while the fries are still hot—the heat will help the seasoning adhere to the fries.

Place the seasoned fries on a warm serving platter. Generously drizzle the warm cheese sauce over the fries, followed by the ranch dressing. Top with chopped green onions or chives, and any other garnishes as desired.

Serve immediately with extra cheese sauce and ranch on the side for dipping.