In a small saucepan, reduce the balsamic over a medium heat until reduced by two thirds. Allow to cool. (You could do a larger quantity, as reduced balsamic is always nice to have on hand.)
Remove the outer leaves of the radicchio until you get to the deep red centre of the head.
Break up the red heart and tear it into uniform bite size pieces. Remove the white spines by cutting a V in each leaf.
Toss the walnuts with the honey, place them in a non stick pan, and cook over a high heat, tossing all the time or they will burn.
Once golden, remove them from the pan and spread out on baking paper and allow to cool.
In a small bowl, mix the mustard with a good splash of olive oil, season with a little salt and pepper, and whisk until combined.
Place the lettuce, watercress and grape halves in a large mixing bowl and toss with a few tablespoons of the dressing.
Arrange evenly on 4 plates and garnish with crumbled cheese, walnuts and a drizzle of the reduced balsamic.