1. Preheat the oven to 400 degrees. Coat a large baking sheet with cooking spray.
2. Mist the chopped sweet potatoes with cooking spray, season to taste with salt and black pepper and bake 25 to 30 minutes or until potatoes are golden brown and tender.
3. Set a grill pan over medium-high heat. Brush both sides of the steak with the oil and season to taste with salt and black pepper. Add steak to pan; cook 5 minutes per side for medium. Let stand 10 minutes before slicing crosswise into 1/4-inch-thick slices.
4. In a food processor or blender, combine the mayonnaise, spinach, mustard and garlic powder. Process until smooth.
5. Serve steak slices with spinach mayonnaise and roasted sweet potatoes.