Place pork tenderloin on a large piece of plastic, slice lengthwise ⅔ of the way through. Open like a book, cover with another piece of plastic and pound flat.
Drizzle both sides with oil and season with salt and pepper to taste. Set aside.
In a small bowl (I use the measuring cup for less dishes) Combine orange juice, mustard, brown sugar and rosemary. Set aside.
Heat a large skillet over medium-high heat. Add pork tenderloin to hot skillet. Cook until first side is well browned, about 5-6 minutes. Flip and cook on 2nd side until pork is just cooked through, about 5 minutes
Remove tenderloin from pan and place on rimmed cutting board and lightly cover with foil.
Add orange juice mixture to skillet along with any accumulated meat juices from cutting board. Add cranberries and cook stirring until reduced by half, about 2-3 minutes. Remove from heat and whisk in butter just until it melts.
Thinly slice meat and place on serving platter. Spoon cranberry orange sauce on top and garnish with additional fresh rosemary.