Prepare the Potatoes: Peel and cut the russet potatoes into thin fries. Rinse them well in cold water to remove excess starch, then pat them dry with paper towels.
Coat the Fries: In a large bowl, combine the flour, Cajun seasoning, garlic powder, onion powder, paprika, salt, black pepper, and cayenne pepper (if using). Dip the potato fries into the buttermilk, then dredge them in the seasoned flour mixture, ensuring they are evenly coated.
Fry the Fries: Heat vegetable oil in a deep fryer or large pot to 350°F (175°C). Fry the coated fries in batches for about 4-5 minutes or until golden brown and crispy. Remove the fries from the oil and drain on paper towels.
Top the Fries: While the fries are still hot, sprinkle them with shredded cheddar cheese, Monterey Jack cheese, crumbled bacon, sliced jalapeños, and chopped green onions.
Serve: Serve immediately while the cheese is melted and gooey. Optionally, drizzle with ranch dressing or your favorite dipping sauce.