Preheat oven to 400°. Coat steak on both sides with 1 tablespoon olive oil and balsamic vinegar. Rub 1 teaspoon pepper and half of rosemary onto both sides; let stand about 30 minutes.
Toss potato with remaining 1 tablespoon olive oil and rosemary, and add sea salt and pepper. Spread in a single layer on a rimmed baking sheet. Bake at 400° for 20 to 25 minutes or until golden brown, stirring occasionally.
Grill bell pepper and onion, uncovered, over medium-high heat, 3 to 4 minutes on each side. Season steak on both sides with sea salt. Grill steak, uncovered, over medium-high heat 4 to 5 minutes on each side. Remove from hottest part of grill, and cook, covered, another 4 to 5 minutes or until temperature registers 130° for medium rare. Remove steak from grill; cover loosely with foil, and let stand 5 to 10 minutes before slicing. (Meat will continue to cook.)
Slice steak into 1/2-inch slices, and toss with potato, grilled vegetables, baby greens, cilantro, and Mustard Vinaigrette. Divide evenly onto plates. Serve warm.