Cut the Brussels sprouts into halves (if large) or quarters (if small) so that they are about the same size as the butternut. I like a 1:1 ratio of squash to Brussels, however adjust to your liking.
Roast on 400°F for 20-30 minutes. About halfway through, stir and add as many cranberries as you like. Continue baking until the squash is tender and browned to your liking.
Alternatively, rather than continuing to bake after adding the cranberries, broil for about 3-5 minutes.
When your butternut, Brussels, and cranberries are done roasting, transfer to a serving bowl. Sprinkle with a pinch of salt and black pepper if needed. Serve with the dressing.
While those are baking, use a whisk to thin out the Dijon mustard with water or olive oil until a pourable consistency. A few spoonfuls of Dijon is a good place to start for the amount. Whisk in about 1/4 the amount of Dijon in lemon juice. Taste and adjust to your liking. If you want a kick, add a pinch of cayenne pepper. Serve over the veggies.
If you don't have butternut on hand, other winter squash like acorn, pumpkin, or kobocha squash, or sweet potato will work.