I'm making a Fennel and Rosemary encrusted Beef tenderloin with a Dijon cream sauce. My guests are low/no carb. Looking amazing salad suggestion?

I agree that a raw fennel salad to echo the fennel seeds sounds pretty darn good - I am kinda stuck on fennel, blood orange and red onion salads lately - soak the onions in cold water to take the bite outt, and I dress them simply in good olive oil, a dash of red wine vinegar, salt and pepper, and some minced fennel frond. It's not heart but beef IS hearty so, maybe yes?

My suggestion for wine is - lots of it.
GAWD I can be helpful at times :-)

I agree that a raw fennel salad to echo the fennel seeds sounds pretty darn good - I am kinda stuck on fennel, blood orange and red onion salads lately - soak the onions in cold water to take the bite outt, and I dress them simply in good olive oil, a dash of red wine vinegar, salt and pepper, and some minced fennel frond. It's not heart but beef IS hearty so, maybe yes?

My suggestion for wine is - lots of it.
GAWD I can be helpful at times :-)

I'm a huge fan of chopped spinach and greens mixed with shaved Brussels sprouts. Topped with bleu cheese, dried (unsweetened) cranberries, almond slivers and a Dijon buttermilk dressing. You can switch up the specifics. If you don't like bleu, try feta. Can't find the unsweetened cranberries, slice Granny Smiths. No tree nuts, get crunch and mimic flavor a with sliced fennel. Too much Dijon, lemon vinaigrette.

How about the Napa Cabbage wedge salad recipe (https://food52.com/recipes/31357-napa-cabbage-wedge-salad-with-apples-and-buttermilk-dressing)? Or the kohlrabi bistro salad recently posted on thekitchn.com?

Sounds like a wonderful main dish.
So, if you want to echo the fennel seed, and have something with bright and sometimes slightly bitter flavor (the watercress) to offset the creamy steak sauce, use one of these:
Fennel & apple from Bonnie Stern - clean & simple flavors, fresh, won't compete with the beef.
http://news.nationalpost.com/appetizer/bonnie-stern-green-crunch-goes-well-with-grey-weather
Artichoke, fennel & arugula salad from Eric Ripert (in WSJ) - use canned or frozen artichokes when they're out of season. nice bitterness.
Shaved fennel, celery & egg salad from Deborah Madison, from her book, Vegetable Literacy, 2013.
Watercress, fennel, orange & avocado from Gail Monohan (also in WSJ).
Among the many red wines that would go with the mustard-cream sauced steak, I suggest a cabernet sauvignon or carmenere or blend of the two from Chile.

For wine I would do a Bordeaux or sangenivese.

I would go with the classic steakhouse favorite "the wedge" iceberg wedge, blue cheese dressing, bacon, and tomatoes (I use sundried tomatoes in oil). Its low carb and tasty.