Beef – How to Reheat Beef Tenderloin

Try wrapping it in foil and cooking on a very low temperature in the oven or toaster oven. (Our oven has a "warm" setting that's ~170 F.) You can also include some water or broth in your foil packet but it won't penetrate much beyond the surface if the meat has been cooked before. It'll make that 1st bite taste more moist but in the long run it won't do much.

As this basically got answered through questions in the comments, I'll collect up all of the various problems that might help prevent the sogginess:

1. After frying, allow the croquettes to cool on a wire rack, so they don't get moist as they give off steam.
2. Reheat first at a lower temperature to heat through, then turn it up the temperature to regain the crunch & get any extra browning.
3. Make sure when reheating that they aren't packed too tightly, and if possible, heat on a wire rack rather than a pan.
4. If reheating in a small but tightly-sealing oven, try cracking the door slightly so as to prevent humidity buildup.

(anyone who has any other suggestions, feel free to add 'em)