Preheat oven to 350. Place the walnuts on a baking sheet and toast in the oven for 10-12 minutes, stirring a few times and watching carefully so they don't burn. Remove from the oven and allow to cool. Set aside.
Increase the oven temperature to 400F and place the beets (with the tops and tails chopped off, but skin left on), in a piece of aluminum foil. Scrunch it up around them so that they are mostly covered. Roast for about 1 hour or until tender, this will depend on the size of your beets.
Remove the beets from the oven, and allow to partially cool so they are comfortable to handle. Remove the skin and chop into chunks about 1 inch big. Set aside to cool.
Make the salad dressing by whisking together all of the ingredients. Taste and adjust as necessary. You may want to start with 1/4 tsp of the allspice and increase if desired.
Toss the salad greens, cooled beets and walnuts with some of the dressing.
Slice the brie into thin rectangular slices, arrange on top of the salad, and top with more of the allspice dressing.