Though tataki is typically made with beef that has been seared on the exterior, but is essentially raw in the center (other than the very brief sear, it's basically carpaccio), it works great with steaks that have been cooked to rare or medium-rare. Slicing the steaks cold will allow you to get the super-thin slices you're after; a sharp knife is still an essential tool to get you there.

For the Dressing:

• 1/4 cup (60ml) homemade or store-bought ponzu
• 1 tablespoon finely minced shallot (about 1/2 ounce; 15g)
• 1 tablespoon (15ml) whole grain mustard
• 1/4 cup (60ml) extra-virgin olive oil
• Small dab prepared wasabi (optional)

To Serve:

• 1/2 pound (225g) leftover steak, sliced as thinly as possible with a sharp knife (see note above)
• 1 small Asian cucumber, thinly sliced (English cukes are also okay; American cukes should be peeled)
• 2 scallions, thinly sliced
• Kosher salt

For the Dressing: Combine ponzu, shallot, mustard, oil, and wasabi (if using) in a bowl and whisk thoroughly.

To Serve: Arrange steak slices evenly over a serving platter. Lay cucumber slices on top. Spoon dressing over the dish (you may not need all of it) and sprinkle with scallions. Season with a little salt. Serve immediately. Leftover dressing can be stored in the refrigerator for up to 1 week.

source: http://www.seriouseats.com/2016/06/leftover-steak-ponzu-mustard-vinaigrette-japanese-salad.html