Roast Beef Sandwiches with Lettuce and Onions
Rinse the roast beef, pat dry, season with salt and pepper and sear until golden brown on all sides in 2 tablespoons hot oil. Remove from the pan, brush with mustard and transfer to a rack set over a roasting pan. Roast in a preheated oven at 140°C (approximately 275°F) for about 30 minutes.
Peel the onions, cut into rings and sweat until golden brown in the remaining oil, stirring occasionally. Season with salt and pepper.
Rinse the lettuce, pat dry and chop coarsely.
Toast the bread. Remove the roast from the oven, let rest for 5 minutes then cut into thin slices. Divide between four slices of toast, season with salt and pepper then add the onion, salad greens and remaining slices of toast. Cut in half diagonally and serve.