Mix the mayonnaise, ketchup or chilli sauce, pickle relish, horseradish, Worcestershire sauce, lemon juice or pickle brine, Dijon mustard, smoked paprika, garlic powder, salt, and black pepper until smooth. Taste and adjust. The dressing should be creamy, tangy, lightly sweet, and sharp enough to cut through the beef and cheese. If it tastes flat, add more pickle brine or horseradish. The horseradish and pickle brine together give the dressing enough sharpness to stand up against the richness of warm corned beef and two layers of melted Swiss — a dressing that's too mild tends to disappear once the sandwich is fully assembled and hot.

Place the sauerkraut in a sieve and press out as much liquid as possible. For an even cleaner sandwich, squeeze it in a clean towel or paper towels. This is a major turning point. Wet sauerkraut will ruin the rye bread and turn the sandwich soggy before the cheese even melts. Pressing rather than simply draining removes considerably more liquid than gravity alone would over the same time — sauerkraut holds onto brine tenaciously, and this more aggressive step is what actually gets it dry enough to sit safely against bread for the full griddling time.

Place the sliced corned beef in a skillet over low heat with 15–30ml water, or warm it gently in a covered pan for 2–3 minutes until hot and flexible. Drain off any excess moisture. Do not put cold corned beef straight into the sandwich unless you want burnt bread and unmelted cheese. The filling needs a head start. A cold, dense pile of meat acts as a heat sink at the centre of the sandwich — the griddle has to work through that cold mass before the surrounding cheese can properly melt, and by the time it does, the bread on either side has often browned well past ideal.

Spread Russian dressing on one side of each slice of rye bread. On 4 slices, layer Swiss cheese, warm corned beef, drained sauerkraut, and another layer of Swiss cheese. Close with the remaining rye slices, dressing-side down. Keep the sauerkraut in the centre, not spilling to the edges. Exposed sauerkraut burns and leaks moisture into the pan. Sandwiching the sauerkraut between two layers of Swiss cheese does double duty — the cheese melts around it and helps seal in any remaining moisture, while also physically containing it away from the bread's edges where the butter and direct heat would otherwise scorch any exposed pieces.

Spread softened butter on the outside of each sandwich. Use an even layer from edge to edge so the rye browns evenly. An even, edge-to-edge layer of butter is what produces uniform browning across the whole slice — thin or missing patches brown more slowly and unevenly than the rest, leaving a mottled, inconsistent crust.

Heat a large skillet, griddle, or cast-iron pan over medium-low to medium heat. Add the sandwiches and press gently with a spatula or a second heavy pan. Cook for 4–5 minutes on the first side, until the bread is deeply golden-brown and crisp. Flip carefully and cook for another 4–5 minutes, pressing gently, until the second side is crisp and the Swiss cheese is fully melted. Do not use high heat. Rye burns before the cheese melts. Medium-low heat is the difference between a proper Reuben and a scorched mess. This moderate heat is calibrated specifically to let the substantial filling — warm beef, two cheese layers, drained kraut — come fully up to melted, hot temperature in roughly the same window the rye needs to develop deep colour, which a hotter pan wouldn't allow.

Transfer the sandwiches to a cutting board and rest for 1–2 minutes. Cut each sandwich diagonally in half. The short rest helps the cheese settle slightly so the cross-section looks clean but still gives a visible pull. Cheese straight off the griddle is at its most fluid; a brief rest lets it firm up just enough that the first cut doesn't send it pouring out, while it's still melted and stretchy enough to show a proper pull.

Stack the sandwich halves slightly offset so the filling is visible. Serve immediately while the rye is crisp, the beef is hot, and the cheese is melted. A Reuben gets worse every minute it sits because steam softens the rye. The hot, moist filling continues releasing steam even after the sandwich leaves the pan, and that trapped steam works directly against the crisp crust the griddling stage just built — serving without delay is what keeps that crust intact.