How to Make It
Stir together vinegar, sugar, 1/4 cup water, and 1 teaspoon of the salt in a small saucepan; bring to a boil over high. Stir in onion; remove from heat. Let stand 15 minutes. Drain, reserving onions and 3 tablespoons pickling liquid.
Meanwhile, preheat grill to high (450°F to 500°F), or heat a grill pan over medium-high. Stir together ground coffee, chile powder, and remaining 1 teaspoon salt in a small bowl; rub evenly over steak. Place steak on oiled grates. Grill, uncovered, until a thermometer inserted in thickest portion of meat registers 130°F to 135°F for medium-rare, 4 to 5 minutes per side. Remove from grill; let rest 5 minutes. Slice against the grain into 1/4-inch-thick strips.
Place potatoes and 1 tablespoon water in a microwavable bowl. Cover with plastic wrap; microwave on HIGH until tender, about 5 minutes. Cut potatoes in half lengthwise. (Discard water in bowl.)
Whisk together oil, mustard, and reserved pickling liquid in a large bowl. Add greens and halved potatoes; toss gently to coat. Arrange mixture on a large platter; top with sliced steak and pickled onion.