In a large saute pan or skillet over medium high heat, melt butter. When butter is sizzling a bit, add mushrooms and garlic and cook for 2-3 minutes or until mushrooms are softened slightly and garlic is fragrant; remove from heat and let cool slightly. Meanwhile, in a large mixing bowl, gently toss roast beef and cheese together, then add the mushrooms and garlic mixture.
Unwrap the puff pastry and lay one sheet out onto a greased sheet of parchment paper. Using a rolling pin, roll out puff pastry to about 12" x 12" in size, then cut in half diagonally to create two triangles.
Place one quarter of the prepared filling onto one side of the puff pastry triangle, leaving about 1/2" of space around the outer edge of the triangle; fold dough over filling. Using a fork, press edges of dough together to seal tightly; brush pastry with egg wash. Repeat with remaining dough and filling.
Bake packets, uncovered, in preheated 375 degree oven for 20-25 minutes, or until nicely golden brown and puffed.