Preheat the grill or a grill pan over medium-high heat. Put the sweet potatoes into a gallon-size resealable plastic bag with the olive oil and season with salt and pepper. Seal the bag and shake well. When the grill is hot, arrange the sweet potato slices on the grill and cook, turning once with tongs, until nicely blistered on both sides, 8 to 10 minutes. Move the sweet potatoes to a warm part of the grill or keep warm on a plate, loosely covered with foil.
Cut each steak in half crosswise and season with salt and pepper. Grill the steaks until brown and crusty on 1 side, 4 minutes. Meanwhile, mix the Worcestershire sauce and mustard in a cup. Turn the steaks over. Brush the tops with the sauce mixture. Cook for 3 minutes more for medium doneness.
Slice the steaks and serve with the sweet potatoes and additional Worcestershire sauce and mustard on the side.