How to Make It
Using a small sharp knife, make a slit on the flat side of each chestnut. Bring a medium saucepan of water to a boil over high heat. Add the chestnuts, return to a boil and cook for 5 minutes. Using a slotted spoon, transfer the chestnuts to a thick kitchen towel and wrap them to keep warm. One by one, cut and peel away the outer shell and dark brown skin.
In a heavy medium skillet, combine the peeled chestnuts, 1 tablespoon of the butter, the fennel seeds, celery, bay leaf and enough water to cover. Bring to a boil over moderately high heat. Reduce the heat to low, cover and simmer without disturbing until the chestnuts are just tender, 20 to 35 minutes. Using a slotted spoon, transfer the chestnuts to a flat plate to dry.
In a medium skillet, melt 2 tablespoons of the butter over moderately high heat. Add the bread cubes and fry, tossing, until golden brown, 3 to 5 minutes. Add the garlic and a pinch of salt and toss over the heat for 10 seconds. Drain on paper towels.
In the same skillet, fry the pancetta over moderately high heat until almost crisp, 4 to 5 minutes. Add the remaining 1 tablespoon butter, the chestnuts and 1/4 teaspoon each of salt and pepper. Cook until the chestnuts are glistening, about 30 seconds.
Just before serving, place the greens in a large serving bowl. In a small bowl, whisk the walnut oil with the vinegar, 1/4 teaspoon salt and 1/8 teaspoon pepper. Pour the vinaigrette over the greens, add the walnuts and toss. Fold in the warm chestnuts and pancetta and scatter the croutons over the top. Serve at once.