1.Place the water and salt in large saucepan; bring to the boil. Gradually add polenta, stirring constantly. Reduce heat; simmer, uncovered, stirring constantly, about 20 minutes or until polenta thickens. Stir in cheese and egg yolk. Spread mixture evenly into oiled deep 19cm-square cake pan, pressing firmly. When cool, cover; refrigerate about 2 hours or until firm.
2.Meanwhile, cut beef into 3cm pieces, toss in flour; shake off excess.
3.Heat oil in large saucepan; cook beef, in batches, until browned all over.
4.Heat butter in same pan; cook onion, garlic, bacon and mushrooms, stirring, until onion softens. Return beef with any juices to pan. Add wine, stock, paste, mustard and thyme; bring to the boil. Reduce heat; simmer, covered, about 1¼ hours or until beef is tender, stirring occasionally.
5.Turn polenta onto board; trim edges. Cut polenta into four squares; cut each square in half diagonally. Cook polenta triangles on heated oiled grill plate (or grill or barbecue) until browned both sides.