1 1/2 loaves brioche, cut into 1-inch cubes
1 small onion, minced
4 stalks celery, minced
20 chestnuts, minced
2 tablespoons butter
1 1/2 cups milk
1 1/2 cups heavy cream
4 teaspoons English thyme, chopped
Salt and pepper to taste
3-by-4-by-10-inch terrine mold
Set oven to 350 degrees. Place the brioche into a large bowl. Sweat the onion, celery and chestnuts in the butter over medium-low heat until the onions are translucent. Add the vegetables to the brioche and mix evenly.
Crack the eggs into a bowl. In a large pot, bring the milk and cream to a boil. Very slowly whisk the hot milk and cream into the eggs. Pour half of the dairy mixture into the bowl with the brioche and toss to coat evenly. Add the thyme. Add enough of the remaining dairy mixture so that the brioche is fully moistened but not floating. Season with salt and pepper.
Spoon the bread pudding into the terrine mold and bake until golden brown on top, about 45 minutes. Check to make sure that the pudding is done by inserting a knife—it should come out clean.