1. Trim off extra fat from pork tenderloin, and spread mustard on one side.
2. Preheat oven to 350 degree F.
3. Grease a baking dish and set aside.
4. Place a frying pan on flame, and place tenderloin upside down in the pan. Spread mustard on the other side. Saute until nice and brown. Flip and cook.
5. In a greased baking dish, transfer the tenderloin.
6. Pop it in oven and cook for 20 minutes. Remove the pan, flip and cook further for 20 minutes or until its internal temperature reaches 155 degree.
7. For sauce, in the same pan, melt butter.
8. Add shallots, stir and cook for 5 minutes or until softened.
9. Stir in Marsala, heavy cream, and mustard in the pan. Turn up the heat, and cook until the mixture reduces by half.
10.Remove tenderloin from oven, tent it with foil.
11. Slice tenderloin and serve it hot with sauce.
Slice tenderloin and put it in the refrigerator to chill. Then, lay them on a serving platter. Top it with lettuce and reheat before you serve it with sauce as entree.