Halve tenderloin horizontally to form 2 approximately 1-inch-thick pieces.
Rub both sides of each piece with black pepper, pressing it into meat.
Grill beef on an oiled rack set 5 to 6 inches over glowing coals 5 to 8 minutes on each side for medium-rare.
(Alternatively, broil beef on oiled rack of a broiler pan about 3 to 5 inches from heat 6 to 7 minutes on each side for medium-rare.) Transfer beef to a cutting board and cool 30 minutes (I actually prefer it warm).
If desired, chill beef, loosely wrapped, at least 2 hours and up to 1 day to facilitate thin slicing.
But beef across grain into very thin slices.
In a large bowl stir together lime juice, fish sauce, sugar, and chilies until combined will (sugar should be dissolved if using).
Add beef, shallots, scallions, coriander, and mint and toss will.
Score cucumber lengthwise with tines of a fork and cut crosswise into thin slices.
Arrange cucumber around edge of a platter and mound beef salad in center.
Garnish beef with coriander sprig and serve with rice.