Melt the butter in a large skillet over low heat. Add the onions and cook low & slow until deeply caramelized, about an hour. Scrape the bottom of the pot often; adding whiskey helps deglaze the skillet and adds a wonderful flavor.
While the onions are caramelizing, preheat oven to 400°.
Arrange 40 small mushrooms on a foil-lined rimmed baking sheet. Cut any larger mushrooms in half for a total of 40 pieces if needed.
Roast mushrooms at 400° for 15 minutes, until they release liquid. Reduce heat to 325° when done.
While mushrooms are roasting, roll each pastry sheet into a 12x12-inch square on a lightly floured surface.
Cover a baking sheet with parchment paper and lay one pastry sheet on top. Set the brie wheel in the center and place strips of parchment paper on all four sides, touching the wheel and covering the pastry. Drape the second pastry sheet over the brie.
Use a sharp knife to make a slice on each side of the pastry square from the edge to the brie wheel. Cut just through the first pastry sheet to the parchment paper. You should now have four quadrants in the first pastry sheet.
Cut each quadrant evenly into four strips, again slicing from the edge to the the brie wheel. Trim and save scraps from the longer corner strips.
Set a mushroom on the end of a strip and stuff with about half a teaspoon of caramelized onions. Tightly roll up until it is snug against the brie wheel. Repeat for all 16 strips, using scrap pastry to patch any holes if needed.
Remove the parchment paper strips and cut four quadrants in the second pastry sheet.
Cut each quadrant evenly into six strips, again slicing from the edge to the the brie wheel. Trim and save scraps from the longer corner strips.
Set a mushroom on the end of a strip and stuff with about half a teaspoon of caramelized onions. Tightly roll up until it is snug against the brie wheel. Repeat for all 24 strips, using scrap pastry to patch any holes if needed.
Brush the entire surface with egg yolk and sprinkle with poppy seeds.
Bake at 325° for 25-30 minutes, until deep golden brown all over.
Carefully cut around the top of the brie wheel. Remove top just before serving.