Preheat your oven to 400–425°F (or prepare your air fryer per package instructions).

Spread the frozen French fries in a single layer on a baking sheet (do not overcrowd) and bake until crisp and golden, about 22–30 minutes, flipping halfway.

While the fries bake, make the cheese sauce: melt the butter in a small saucepan over medium heat.

Whisk in the flour and cook for 1–2 minutes until bubbly.

Slowly pour in the milk while whisking constantly to avoid lumps.

Continue to whisk until the mixture begins to thicken, about 3–5 minutes.

Remove the saucepan from heat and immediately stir in the shredded cheese until fully melted and smooth.

When the fries are done, remove them from the oven and immediately toss them with the Cajun seasoning.

Transfer seasoned fries to a serving platter.

Drizzle warm cheese sauce over the fries.

Finish with the ranch dressing drizzle and garnish with chopped green onions or chives if desired.

Serve immediately while fries are hot and crispy.