Pierce the potatoes with a fork several times. Microwave on High, turning twice, for 6 to 8 minutes or until fork-tender. Let cool for 10 minutes. Cut each potato into 6 wedges.
Meanwhile, brush the steak with 2 tbsp (25 mL) of the dressing; sprinkle with pepper.
Preheat the oven to 300°F (150°C). Heat a large nonstick skillet set over medium-high heat. Sear the steak on each side for 1 to 2 minutes or until browned. Transfer to a greased baking sheet.
Working in batches; add half of the margarine and potatoes to the skillet; brown on all sides. Transfer to the baking sheet with the steak. Repeat with remaining potatoes and margarine.
Place steak and potatoes in oven, roast for 10 minutes until potatoes are tender and steak is cooked to preferred doneness. Let steak rest for 5 minutes; slice thinly across the grain.
Gently toss the sliced steak with the potatoes, spinach, red pepper, onion and remaining dressing.
Top with shaved Parmesan and serve with lemon wedges (if using)