Arrange an oven rack in the center position and heat oven to 350 degrees. Scatter the bread cubes across a large rimmed baking sheet and bake, tossing cubes every 10 minutes, until dry and golden brown all over, 25 to 30 minutes. Let cool for a few minutes on the baking sheet, then transfer to a large bowl.

Heat a large skillet over medium-high. Add 6 tablespoons butter and swirl until melted, then add the sausage and cook, breaking it up into small pieces with a wooden spoon or spatula and stirring often, until it’s browned and some of the fat has rendered, 6 to 8 minutes. Remove the skillet from the heat and use a slotted spoon to transfer just the sausage to the bowl with the bread, leaving the fat behind.

Return the skillet to medium heat and add the celery, leeks, onion and sage; season with salt and pepper. Cook, stirring often, until the vegetables are soft and browned at the edges, 12 to 15 minutes. Remove the skillet from the heat and scrape the mixture into the bread mixture.

In a medium bowl, whisk the heavy cream and eggs until no streaks remain, then whisk in 2 cups stock. Pour the egg mixture over the bread, then gently mix everything until all the ingredients are thoroughly distributed. Drizzle the remaining 1 cup stock over the mixture a couple of tablespoons at a time, mixing it in between additions, just until the cubes of bread are moistened completely but no liquid pools in the bottom of the bowl. (You may not use all of the stock.)

Smear 1 tablespoon butter across the bottom and sides of a 3-quart baking dish, then add the stuffing mixture in an even layer, pressing to compact it slightly. Cut the remaining 1 tablespoon butter into several small pieces and arrange them on the surface of the stuffing. Cover the dish tightly with foil. (The unbaked stuffing, covered with foil, can be refrigerated for up to 2 days.)

Transfer to the oven and bake for 30 minutes, then remove the foil and increase the oven temperature to 400 degrees. Continue to bake, uncovered, until the stuffing is crispy and browned all over, another 35 to 45 minutes. Any leftover stuffing will keep in an airtight container in the refrigerator for 3 days.