Tip:To adapt a slow-cooker recipe to a conventional oven, follow these guidelines: add more liquid, to accommodate for greater evaporation; bring the dish to a boil over high heat in a Dutch oven, then cover the pot and put in a 350°F oven. Plan on the dish taking roughly half the time to cook in the oven as it would in the slow cooker.
In a large bowl, coat the chicken with the flour, shaking off and reserving the excess. Sprinkle the chicken all over with a total of 1 tsp. of the salt, 10 grinds of the pepper, and the nutmeg. Place the bacon in a single layer in a cold, 10-inch, heavy sauté pan. Heat over medium and cook until most of the fat renders, about 9 minutes. Raise the heat to medium high and cook until golden brown and a bit crisp, stirring, about 6 minutes. Transfer the bacon to the slow cooker (don’t leave any bacon bits in the pan).
Add half of the chicken to the hot pan and cook until lightly golden brown on both sides, about 10 minutes. Repeat with the remaining chicken, about another 5 minutes. Transfer to a large plate. Pour all but about 2 Tbs. of the bacon fat into a small bowl and reserve. Add the celeriac, carrots, onions, and garlic to the hot pan, and cook, stirring, until slightly softened, about 7 minutes. Transfer the vegetables to the slow cooker and top with the chicken.
Add another Tbs. of bacon fat plus the tomato paste and any remaining flour to the sauté pan. Cook, stirring, until no flour is visible, about 30 seconds. Remove the pan from the heat, carefully add the brandy, then return the pan to the heat and bring to a boil over high heat. Boil, whisking, until the sauce is smooth and a bit thick, about 2 minutes. Stir in the stock and honey, and cook for another 2 minutes, whisking, until the sauce is smooth. Pour over the chicken. Cover and cook on low until the chicken is tender, 4 to 4-1⁄2 hours.
Meanwhile, bring a covered pot of heavily salted water to a boil. When boiling, add the Brussels sprouts and cook over high heat until they’re tender but still have a slight bite, about 7 minutes. Set aside. Heat 2 Tbs. of the bacon fat and the butter in a 10-inch, heavy sauté pan over medium-high heat. When the butter is melted, add the mushrooms, the remaining 1⁄4 tsp. salt, and 4 grinds pepper, and sauté until browned and tender, about 9 minutes.
Remove the chicken from the slow cooker, mix in the cooked vegetables (let them sit in the slow cooker for about 10 minutes if they’ve cooled), and add back in the chicken. Serve.