I have made this with leftover steak - it's very simple, but deceptively good. The bread should good quality (not supermarket sliced) but softer than crusty baguette halves - with bits of cold steak crusty bread can make the whole thing a bit too tough. I didn't think to photograph it when I made it but it's a pretty sandwich, too! —LucyS
slices good-quality bread, such as a boule, toasted (see note)
garlic clove, peeled and cut in half
arugula, mizuna, or similar greens, or a mixture (preferably not entirely lettuce)
enough thinly sliced steak to cover one piece of bread
Rub the warm, toasted bread with half a garlic clove. Angle the garlic clove slightly so you're using the edge of it, which will make the bread especially garlicy.
Depending on the strength of your horseradish sauce, coast one or both pieces of toast with a very thin layer.
Coat both pieces of toast with mayonnaise. It will combine with the horseradish and garlic nicely, so don't worry if it seems like there's a lot of stuff on there.
Layer the sliced steak on the horseradish side of the bread. Pile greens on top of it. If the steak is a bit tough, be sure to slice it as thin as you can.
Squash the two sandwich halves together. This sandwich travels very well but if you're going to take it as a bagged lunch, make sure to bind it tightly (I usually use a ziplock that I squash all the air out of) so it doesn't fall apart.