1 (16 oz.) package frozen white pearl onions, thawed
2 tablespoons fresh tarragon, chopped
HEAT oven to 350°F. Place sheet of heavy-duty aluminum foil in a shallow roasting pan with foil extending 6 inches beyond pan on either side. Spray with no-stick cooking spray. Place the brisket fat-side down in middle of foil.
COMBINE dry onion soup mix with jelly and cocktail sauce in medium bowl. Pour half of mixture over the brisket; turn brisket fat-side up, cover with remaining onion mixture. Bring both sides of foil to the middle and fold foil down on itself several times. Repeat with the two remaining sides. Bake 3 hours or until fork-tender. Remove from oven. (Brisket can be cooled and held in the refrigerator overnight at this stage.)
MELT butter in 10-inch skillet over medium heat; cook and stir onions and mushrooms for 5 to 8 minutes or until light golden brown.
REMOVE brisket from pan; pour off juices, skimming the fat off the top. Remove layer of fat from brisket. Slice meat across the grain into 1/4 inch thick slices. Return meat to roaster. Top meat with mushroom mixture, meat juices, and tarragon. Cover with foil. Return to oven to heat through before serving.