Combine the sauce ingredients. Heat a panini press, cast-iron skillet or griddle over medium to medium-high heat.
On a work surface, spread the horseradish sauce liberally over 4 slices of bread. Arrange 2 slices of rare roast beef over the sauce. Combine the red onion (or shallot) with the parsley, chile pepper and lemon zest and sprinkle liberally over the roast beef. Arrange a layer of watercress over the beef. Divide the cheese evenly among the sandwiches and set the remaining bread slices in place. Press the panini 2 at a time on the panini press or in a hot skillet with a pan weighted on top.