Looks like we’re having rack of lamb for Christmas. I will probably do a rosemary and oregano rub for it.
I really want mashed potatoes, but am unsure of other sides.
I am thinking seriously about Leek Bread Pudding as a stuffing-esque dish (I feel like it’s not a winter feast without stuffing of some sort).
What vegetable? We are fond of most vegetables, including beets and Brussels sprouts, both of which are nice and wintry and festive.
What would you serve with rack of lamb for Christmas?
I would keep your veg simple so as not to compete with your rack of lamb and leak bread pudding, both of which sound exquisite. Agree with the fittingness of Brussels sprouts. I’d steam them three quarters done, then half them through the stalk, and finish with a saute in light oil, high heat to sear and carmelize the cut surface while keeping them al dante to add a counterpoint in texture to the softness of the potatoes and pudding. Done so they wouldn’t need anything else, except maybe a light spinkle of salt and pepper, if you wish.
An alternative could be green beans, likewise sauted on high heat, but no presteam. These would probably be crunchier and juicier, but not as seasonally fitting.
Sounds good. I love brussel sprouts, particularly baby ones. If I don’t have them just steamed I toss them in butter with either chopped almonds or pistachios. They are also great shredded, raw as base for coleslaw instead of cabbage.
Two votes for Brussels sprouts. The dope is an odd place. 
Brussel sprouts can be sauteed with shallots (slice the sprouts lengthwise into slices, sautee the shallots, add sprouts and some veggie broth, cook until tender.)
Also, simple grilled asparagas works fine with lamb.
I know you want mashed potatoes, but perhaps consider Roasted New Potatoes
Not a huge fan of brussel sprouts, myself, so the green beans or fresh asparagus (if you can get it) would be my pick.
Mashed potatoes is what first came to my mind, so you are good there. Second the brussel sprouts. So jealous since lamb is my favorite meat but haven’t converted the rest of the family enough to have it for Christmas dinner :(. I like my brussels a little crisp, so I roast them in olive oil and something vinegar (balsalmic, apple, red wine - depends on my mood).
Some kind of Middle Eastern rice dish instead of mashed potatoes.
A nice Greek style salad, as long as it had olives and feta cheese.
Yeah, I’ll vote for Brussels sprouts. They’re my second go-to veg, after asparagus.
Brussel sprouts come from the devil’s ballsack. How about rice pilaf or saffron rice instead of potatoes?
I would do asparagus in lemon butter and cous cous with mixed Moroccan spices. The Moroccan spices come in a mix from the spice section at the grocery. If the potato and the cous cous are too much starch, I would skip the potatoes.
Don’t know about sides, but the last time my sister made a rack of lamb, she threaded it with rosemary and it was awesome.
Thanks for all the suggestions so far. The flavor profile for this meal will be solidly north/western European so mashed potatoes are a must. No pilafs, Middle Eastern couscous, etc. 
I’d go with Pan Roasted Root Vegetablesand some good bread to sop up the juices. Waste none of the lamby goodness