Place noodles in large heatproof bowl and cover with boiling water. Stand for 3 minutes, then drain.
Sprinkle polenta over base of oiled deep 19cm-square cake pan. Press cooled noodles firmly into pan. Cover and refrigerate 3 hours or overnight.
Turn noodle cake onto board and cut into quarters. Heat oil in large frying pan and cook noodle squares, in batches, until browned lightly on both sides. Drain on absorbent paper and cut each square into two triangles
Slice mange tout diagonally; cut beans into 5cm lengths. Boil mange tout and beans, separately, until just tender and drain. Cut onions into long thin strips.
Toss peas, beans, sweetcorn, peppers and onion in large bowl with the duck. Divide noodle wedges among plates.
Combine dressing ingredients in screw-top jar; shake well.