Ask your butcher to butterfly the flank steak, or you can do it yourself: Lay meat on a cutting board with the grain running vertically. Using a long, sharp knife, cut the meat almost in half, opening it like a book but stopping before going all the way through. Cut several 12-in. pieces of kitchen twine and space evenly under steak.*
Toss cornbread stuffing with 1 cup boiling water and 1/2 cup salsa until moistened; stir in cilantro. Spread over steak, leaving about 1 inch along edges. Roll steak starting from a long side, jelly-roll style; tie with twine.
Place seam side down in a 6-qt oval slow-cooker. Spoon remaining 1/2 cup salsa over meat. Cover and cook on low 8 to 10 hours.
Remove steak to a cutting board; let rest 10 minutes. Discard cooking liquid. Remove string and cut steak into 1/2-in.-thick slices. Serve with additional salsa, if desired.