- With the rack in the middle position, preheat the oven to 400°F (200°C).
- In a baking dish, spread the leeks in an even layer and cover with the cream and butter. Season with salt and pepper. Cover and bake for 40 minutes. Uncover and continue baking for about 5 minutes. Set aside.
- Meanwhile, in a skillet, brown the meat on both sides in the oil until the desired doneness. Set aside on a plate and let rest for about 15 minutes. Thinly slice the meat.
- With the rack in the highest position, preheat the oven’s broiler.
- Place the bread slices on a baking sheet. Spread with the mustard and cover with the cheese. Broil for about 5 minutes or until the cheese is melted and the bread begins to toast. Garnish with the sliced beef, leeks, and arugula. Generously season with pepper.
The sandwiches are also delicious made with roast beef leftovers.