In 10" skillet over medium-high heat, cook pork sausage and mushrooms until meat is well browned and mushrooms are tender, stirring occasionally. With slotted spoon, remove sausage mixture to paper towels to drain. In large bowl, mix sausage mixture and shredded cheese.
With knife, cut phyllo lengthwise into 2" wide strips. Place strips on waxed paper; then cover with slightly damp paper towel to prevent phyllo from drying out.
Place 1 strip of phyllo on work surface; brush top lightly with melted butter. Place about 1 teaspoon sausage mixture at end of strip. Fold one corner of strip diagonally over filling so that short edge meets the long edge of the strip, forming a right angle. Continue folding over at right angle.
Continue folding over at right angles until you reach the end of the strip to form a triangular-shaped package. Place package, seam-side down, in 15 1/2" X 10 1/2" jelly-roll pan; brush with butter. Repeat with remaining phyllo strips and sausage mixture. If not serving right away, cover with foil and refrigerate.
To serve, preheat oven to 425oF. Bake triangles 15 minutes or until golden. Makes about 3 1/2 dozen hors d''oeuvres. Each triangle: About 50 calories, 4 g fat, 5 mg cholesterol, 90 mg sodium.
TO FREEZE AND USE UP TO 1 MONTH LATER: Place unbaked triangles in freezer container with waxed paper between each layer; seal; label: and freeze. About 35 minutes before serving, preheat oven to 425oF. Arrange frozen triangles on un-greased jelly-roll pan and bake 20 minutes or until triangles are golden.