Place brisket on a large piece of heavy-duty aluminum foil; sprinkle both sides evenly with soup mix. Tightly fold foil to seal; place brisket in a large roasting pan.
Bake at 350° for 4 to 5 hours. Remove brisket from foil, reserving pan drippings. Let brisket cool slightly.
Spoon strained pan drippings over brisket; cut diagonally across the grain into thin slices. Serve with flour tortillas and salsa.
