Preheat your oven to 350 degrees.
Melt the butter in a skillet on medium heat. Add the onions, and cook for 5 minutes. Stir in the dijon mustard, Worcestershire sauce, & poppy seeds and cook until the onions are soft (about 10 more minutes).
Slice the rolls in half all at once & place the bottom half in a 9×13 casserole dish or lined baking sheet. Spread on ¾ of the onion mixture. Top with the turkey, spreadable brie cheese, & cranberry sauce.
Top with the other half of the buns. Spread the remaining onion mixture on top of the buns.
Cover with aluminum foil & bake in the preheated oven for 15-20 minutes. Remove from the oven, and slice down the seams to make individual sliders.