Step 1: Peel and cut potatoes into thin, fry-sized strips. Rinse in cold water to remove starch, then pat them super dry with paper towels.
Step 2: In a large bowl, whisk together flour, Cajun seasoning, garlic powder, onion powder, paprika, salt, black pepper, and optional cayenne pepper. Dip dry potato fries into buttermilk, letting excess drip off. Immediately dredge in seasoned flour mixture, ensuring each fry is fully coated. Do this in batches to avoid clumping.
Step 3: Pour enough vegetable oil into a deep fryer or heavy-bottomed pot to submerge fries. Heat oil to 350°F (175°C). Carefully add coated fries in batches, ensuring not to overcrowd. Fry for about 4-5 minutes per batch, or until golden brown and crispy. Use a slotted spoon to remove fries and drain on paper towels.
Step 4: As soon as a batch of hot fries comes out and drains for a second, sprinkle generously with shredded cheddar cheese, Monterey Jack cheese, crumbled bacon, sliced jalapeños, and chopped green onions. The heat will melt the cheese. Repeat for each batch.
Step 5: Serve immediately while hot, crispy, and the cheese is melted.