Preheat the oven to 350ºF.

Make the salad: Soak, drain, and dry the radicchio according to the directions in Radicchio Slaw with Warm Honey Dressing. In a large bowl, combine the radicchio and watercress. Combine the lemon juice, honey, salt, and pepper to taste in a small skillet. Cook over moderate heat, whisking until the honey dissolves. Remove from the heat and whisk in the olive oil. Taste and adjust the seasoning. Let cool to room temperature.

Make the polpettone: Put the turkey in a bowl. Add all the remaining ingredients except for the mozzarella and work them in gently by hand. Divide the mixture into 8 equal pieces. Flatten each piece into a round patty. Top 4 of the patties with a piece of mozzarella and then with a second patty. By hand, press the edges together to seal them and to make patties of even thickness.

Make the coating: Put the seasoned flour and the bread crumbs on separate sheets of waxed paper. Crack the egg into a shallow bowl and beat lightly.

Coat the patties lightly with the flour, then dip in the beaten egg, and finely in the bread crumbs, pressing the crumbs gently into place.

Heat a large ovenproof skillet over moderate heat. Add olive oil to a depth of 1/8 inch. When the oil is hot, add the 4 patties. Cook on one side until nicely browned, about 1 1/2 minutes, then turn and cook on the second side until browned, 1 to 2 minutes longer. Transfer to the oven and cook until done throughout, about 10 minutes.

While the polpettone bake, slice the apple crosswise into paper-thin slices (a vegetable slicer is handy for this). Add the apple to the radicchio and watercress. Toss with enough dressing to moisten the salad, which should leave you some dressing for garnish.

To serve, divide the salad among 4 plates, mounding it attractively. Put 1 "meatball" on each plate, leaning it against the mound of salad. Drizzle the remaining dressing over the polpettone. Serve immediately.