How to Make It
Preheat oven to 425°F. Place sweet potato slices on an aluminum foil-lined large rimmed baking sheet. Drizzle with 1 tablespoon oil, and sprinkle with 1/4 teaspoon each of salt and pepper; toss to coat. Spread potatoes in a single layer. Bake in preheated oven until lightly browned and tender, 12 to 15 minutes. Cool 5 minutes.
Meanwhile, brush steak with 1 tablespoon of the oil, and rub with 1 teaspoon of the salt and remaining 3/4 teaspoon pepper; let stand 10 minutes. Heat a grill pan over medium-high; brush with 1 tablespoon of the oil. Cook steak 6 to 7 minutes per side for medium-rare, or to desired degree of doneness. Remove steak from pan, and let stand 10 minutes. Cut steak across the grain into thin slices, reserving steak for sandwiches the following day, if desired.
Divide baby greens, arugula, red onion, blue cheese, potatoes, and sliced steak among 4 serving plates. Whisk together vinegar, mustard, and remaining 1/4 cup oil and 1/4 teaspoon salt; drizzle over salads.
Cut foccacia bread in half horizontally; slather cut sides with mayonnaise. Layer reserved steak with arugula, blue cheese, and sliced red onion on bottom half, and cover with top of bread.