Preheat the grill to medium-high. Brush the garlic from the steak, then grill 3 minutes per side for medium rare. Transfer the steak to a cutting board, let rest for 5 minutes, then slice thinly against the grain.
Halve the potatoes lengthwise. In a large bowl, toss the potatoes with the arugula, 1/8 teaspoon salt, the remaining 2 tablespoons olive oil, and the lemon juice. Transfer to a serving platter, add the avocado slices, sprinkle with salt, top with the steak, and serve with the salsa.