Preheat oven to 375 degrees without the fan. Line a baking sheet with parchment paper. Lay thawed pastry sheet on a lightly floured work surface. Trim and discard a very thin bit of dough from all edges, so that the layers puff up when baked. With a rolling pin, quickly roll dough out with a few passes to make a 12" x 6" rectangle (does not need to be exact; do not overwork dough). Using a fork, dock (poke) dough approx. 12 times around the center. Brush borders generously with beaten egg; fold edges in about 1/2" and press down gently to seal. Transfer to lined baking sheet; brush entire tart shell with egg. Bake 12 min, rotate 180 degrees and bake an additional 6-8 min, until puffed high and deep golden brown. Remove from oven and transfer to a cooling rack on parchment paper to avoid cracking shell. Let cool completely. Melt together 2 T honey and butter and brush over entire cooled shell. Carefully collapse the domed center of the pastry by gently pushing down to a uniform thickness. Spread with Mascarpone Filling and top with mixed berries. Drizzle the remaining honey and sprinkle with mint. Best served fresh, or chill a few hours until ready to serve.
In mixer bowl, combine mascarpone and sugar on medium speed until smooth, approx. 1 min. Add remaining ingredients and beat 1 min more until fluffy. Chill until ready to use.