I guess I'm just not awake yet....but I can't imagine actually flattening a beef roast like a chicken breast.......there is a method of slicing the raw beef...starting with fat side down....and cutting about an inch or two all the way through the roast so that you have an elongated peice of beef....you can then cover the meat with a filling ( I've done this with an onion flavored bread stuffing) then rolling it much like a jelly roll, searing the rolled roast, then finishing cooking in the oven. Delicious. I just can't think of the name of the method. Probably something SIMPLE like rolled roast beef!?!?
Classic Fillet Mignon . . .
From a beef Tenderloin, cut a piece about 4 - 6 inches in length.
Stand the piece on your board and press it down to about two thirds (maybe even half) of its original height.
Proceed with your Fillet Mignon.
Most available Fillet Mignon is cut to the end-cooking thickness, which won't yield the same mouth feel and flavour experiences that a classic preparation will give the dinner. Some of the benefits of roasts are advantages of thickness, mass, retention of moisture, retardation of the speed of heat penetration . . .
Other than this, I think a flattened Beef Roast might be counter-productive.