Saucy Steak Sandwiches

Directions

1. Sear steak in a skillet or grill pan to medium rare, about 3 min per side. Set aside. 
2. In large skillet, melt butter. Cook onions with salt and pepper until golden brown, stir in broth and mustard and cook until most of the liquid has been absorbed, about 5 minutes. 
3. Slice steak across the grain and stir into hot onion mixture. 
4. Arrange rolls cut side up on a baking sheet. Divide steak mixture over the bottom half of the rolls. Top with Swiss cheese. Place under the broiler for 1 minute or just until cheese is melted and bubbling
5. Top with arugula and cover with bun tops to make sandwiches