Wash and peel the russet potatoes, then cut them into thin strips. Soak the fries in cold water for 30 minutes, then drain and pat dry.
In a large bowl, combine paprika, garlic powder, onion powder, cayenne pepper, salt, and black pepper to create the Cajun seasoning.
Toss the dried fries with vegetable oil, then spread them in a single layer on a baking sheet lined with parchment paper. Bake at 425°F for 25-30 minutes, flipping halfway through.
Remove fries from the oven, sprinkle with cheddar and feta cheese, and return to the oven for an additional 5 minutes until cheese is melted. Top with green onions and drizzle with ranch dressing.