Melt butter in a 12-in. fry pan over medium-high heat.
Add onions and sauté until just tender, about 8 minutes.
Add minced thyme, reduce heat to medium and cook until onions are golden, stirring often, about 15 minutes.
Add honey and vinegar and continue sautéing until soft and brown, about 10 minutes.
Season the onions to taste with salt and pepper. Set aside to cool.
Slice Brie in half horizontally. Spread the caramelized onions in between the cut sides of Brie and sandwich the two pieces together. Wrap puff pastry around Brie, sealing edges with egg wash and trimming any excess pastry.
Place wrapped Brie on a parchment-lined baking sheet. Wrap with plastic wrap and let chill in refrigerator for at least 45 minutes.
Preheat oven to 400°F/205°C. Remove from refrigerator, coat with egg wash.
Place in oven and bake until pastry is deep golden and puffed, about 20 - 25 minutes. Let rest 5 minutes before serving.
Sprinkle with almonds. Transfer wrapped Brie to a platter, and surround with crackers and baguette slices.