OP here. Thank you all so much, these are great suggestions!

I'll let you know what I decide to do.

I was going to say creamed spinach but a PP already suggested it - it's wonderful!

Dessert: we do a massive, varied cookie tray.

I second the root vegetables. Delish and the perfect complement to the beef. Here's a super easy recipe for white chocolate bread pudding. You can make in individual ramekins which looks great - or one large one.

http://www.cakespy.com/blog/2013/9/9/new-orleans-bread-pudding-palace-cafe-recipe.html

Broccoli.

How about a light sponge cake?

I would roast some root vegetables like parsnips and carrots with the tenderloin. Delicious and adds a bit of color. (Depending on the recipe you're using to make the meat, you may have to start the veggies in the oven first. Root veggies take about 40 minutes to roast.)

I think triffles look festive--they don't have to have fruits all perfectly symmetrical. You can also do individual ones, I think this gingerbread one looks intriguing and seasonally appropriate:
http://www.tasty-trials.com/2012/11/gingerbread-trifle-procrastination-and.html

Asparagus or sautéed spinach or chard would be my choice. Both are fast and look elegant. My mom used to do creamed spinach with tenderloin- so rich but so good.

Dessert- something classic. Chocolate mousse or cake, cheesecake, bananas foster even. Bonus points for the bananas- it's quick and easy and all you have to store is ice cream.

We are having beef tenderloin for dinner on Christmas Day, with fingerling potatoes, salad, a vegetable of some kind, and rolls.

I need to come up with:

*vegetable (green beans are out as my son is allergic to them)
*dessert

Any suggestions? What are you having for dinner?

Thanks.