Cook the couscous according to package instructions – do not use any seasoning packet that may come with the couscous. Cook the steak to your preference, then set aside to rest. (For a 2" steak, that takes a mere 3 - 4 minutes per side in a ripping hot cast iron pan with a little oil, or same timing on a hot grill. I shoot for 125° F internal temperature, then allow to rest off heat.)
Mix the dressing in your salad serving bowl. Start with a dab of Dijon mustard (1/2 teaspoon or so), a few pinches of salt, and some sherry vinegar (about 2 teaspoons for 1-2 servings). While whisking, drizzle in about twice as much olive oil as vinegar. Taste (with a leaf of arugula) and adjust to your preference with more vinegar or oil. Add a few grinds of fresh black pepper.
Put the arugula in the salad bowl with the dressing, add the couscous, avocado, drained raisins, and capers and toss to combine. Plate and top with parmesan and steak slices.