Scrub the russet potatoes well. Peel if preferred. Cut each potato lengthwise into 1/2-inch thick planks, then slice planks into 1/2-inch sticks. Keep cut fries in a large bowl of cold water as you work to prevent browning.

Soak

Place the cut potatoes into the 4 cups of cold water and soak for at least 30 minutes, up to 2 hours. This removes surface starch for crisper fries. If short on time, soak 15 minutes.

Rinse and dry

After soaking, drain and rinse the fries thoroughly under cold running water until water runs clear. Spread the fries on clean kitchen towels or paper towels and pat completely dry. Removing moisture is essential to prevent oil splatter and to get crisp fries.

Optional toss with cornstarch

If using cornstarch, place dried fries in a large bowl, sprinkle 1 teaspoon cornstarch over them, and toss until lightly and evenly coated. This helps create a crisper exterior.

Par-fry (first fry)

Heat oil in a heavy pot or deep fryer to 325°F (160°C). If you don't have a thermometer, heat until a small piece of bread sizzles gently. Working in batches, fry the potatoes for 3–4 minutes until they are just cooked through but not brown. Use a slotted spoon to remove them to a wire rack or paper towels to drain. Let them rest 10 minutes. Maintain oil temperature between batches.

Final fry

Increase oil temperature to 375°F (190°C). Fry the par-cooked fries in batches for 2–3 minutes or until golden brown and crisp. Remove to a wire rack or paper towels and immediately season with kosher salt, black pepper and smoked paprika (if using). Keep fries warm in a low oven (200°F / 95°C) if necessary while finishing remaining batches.

Grilled Cheese Sandwiches

Assemble sandwiches

Lay out 8 slices of bread. On four slices, spread a thin layer (about 1/4 tsp) of Dijon mustard if using. Divide the grated cheddar and mozzarella between the four sandwiches, placing about 1/2 cup cheddar and 1/4 cup mozzarella per sandwich for a total of roughly 3/4 cup cheese per sandwich. Season the cheese layer lightly with 1/8 tsp salt and a pinch of black pepper per sandwich. Top with the remaining bread slices to form sandwiches.

Butter the exteriors

Spread about 1 tablespoon (equivalent to 1/2 tbs) of softened butter on the outside of each slice of bread (both top and bottom) OR spread mayonnaise (~1/2 tbs per slice) on the outsides for an extra golden crust. Press sandwiches gently to compact.

Cook

Heat a large nonstick or cast-iron skillet over medium-low to medium heat. Place 2 sandwiches in the skillet (do not overcrowd). Cook slowly 3–4 minutes per side, covering the pan with a lid for the first 1–2 minutes of the first side if you want extra melt—this traps heat and helps the cheese melt before the bread over-browns. Flip the sandwich when the bottom is deep golden brown and continue cooking until the other side is golden and the cheese is fully melted. Adjust heat as needed to avoid burning—lower heat for a longer time yields a better melt. Repeat with remaining sandwiches.

Rest and slice

Transfer cooked sandwiches to a cutting board and let them rest 1 minute to avoid hot cheese oozing out. Slice each sandwich in half diagonally and serve immediately alongside fries.

Dipping Sauce (optional)

Combine ketchup, mayonnaise, Worcestershire sauce and hot sauce (if using) in a small bowl. Whisk until smooth. Taste and adjust seasoning. Serve chilled or at room temperature with fries.