Wash and scrub the potatoes. Leave the skin on for texture or peel if you prefer. Cut each potato lengthwise into 1/3-inch thick planks, then into 1/3-inch sticks to make fries. Place cut fries in a large bowl of cold water and soak for at least 30 minutes (or up to 2 hours) to remove excess starch.

Dry and toss

Drain the soaked fries and spread them on clean kitchen towels or paper towels. Pat completely dry. If using cornstarch, place fries in a large bowl, sprinkle with 1 tablespoon cornstarch and toss to coat evenly; this helps achieve extra crispiness. Season lightly with 1/2 tablespoon kosher salt and 1/2 teaspoon ground black pepper.

First fry (blanch)

Heat vegetable oil in a heavy pot or deep fryer to 325°F (163°C). Fry the potatoes in batches (do not overcrowd) for 3–4 minutes until soft but not colored. Remove with a slotted spoon and drain on a wire rack or paper towels. Let cool for 10 minutes.

Second fry (crisp)

Increase oil temperature to 375°F (190°C). Fry the blanched fries in batches for 2–3 minutes until golden brown and crispy. Remove and drain on a rack or paper towels. Immediately sprinkle remaining 1/2 tablespoon kosher salt over the hot fries and a pinch of ground black pepper. Transfer fries to a serving platter or an oven-safe pan kept warm at 200°F (95°C) while you make the cheese sauce.

Cheese Sauce

Make roux

In a medium saucepan over medium heat, melt 3 tablespoons unsalted butter. Once melted and foaming, add 3 tablespoons all-purpose flour and whisk continuously for 1–2 minutes until the mixture is bubbly and fragrant but not browned (this cooks out the raw flour taste).

Add milk

Gradually whisk in 1 cup whole milk, a little at a time, ensuring the mixture is smooth. Continue cooking and whisking until the sauce thickens and just starts to simmer, about 3–4 minutes.

Finish sauce

Reduce heat to low. Remove the saucepan from direct heat and stir in 1.5 cups grated sharp cheddar and 1/2 cup grated Monterey Jack until melted and smooth. Stir in 1 teaspoon Dijon mustard, 1 teaspoon hot sauce, 1/2 teaspoon salt, and 1/4 teaspoon freshly ground black pepper. If the sauce is too thick, add up to 2 tablespoons more milk to reach desired pourable consistency. Keep warm over the lowest heat or in a warm place, stirring occasionally.

Assembly & Toppings

Assemble

Arrange the hot fries on a platter or individual plates. Pour the warm cheese sauce evenly over the fries, using as much as desired so fries are well-coated but not swimming.

Add toppings

Sprinkle chopped green onions and crumbled crispy bacon over the sauced fries. Add sliced pickled jalapeños if using for heat. Dollop sour cream on the side or in small spoonfuls on top.

Serve

Serve immediately while fries are hot and sauce is warm. Provide napkins and optional ketchup or ranch on the side.