Turn thin tail of beef under and tie with string.
Place butter in a pan and roast beef in a 425 deg. oven for about 40 min.
Meanwhile, make Duxelles and set aside.
Remove beef from oven and cool.
Open rolls per package directions and carefully unroll dough, but do not separate rolls.
On a lightly floured sheet of waxedpaper, arrange dough pieces to form a rectangle.
Pinch perforations together to make 1 piece of dough.
Place Duxelles on the dough.
Place beef, top side down, on top.
Bring edges of dough together down the center of the beef.
Brush edges of dough with eggwhite and pinch to close, forming a smooth seam.
Place seam side down in shallow pan or on a cookiesheet.
With remaining dough, cut flowers or leaves; moisten with egg white and arrange over top of dough.
Brush entire surface with egg white.
Bake in a 400 deg. oven for about 25 minutes or until crust is browned.
Makes 6 to 8 servings.
Put the mushrooms in a dish towel, a few at a time and squeeze out the moisture.
In a skillet, heat butter and oil; sauté onions.
Add mushrooms and sherry and cook until dry.
Season with salt and pepper.
Makes about 2/3 cup.