How to Make It
In a large ceramic or glass bowl, combine the chicken with the wine, chopped onions, carrots, Cognac, olive oil, garlic, peppercorns and 1 teaspoon of salt. Cover and refrigerate for 1 day.
Preheat the oven to 350° Drain the chicken and vegetables in a colander; reserve the liquid. Separate the chicken and vegetables and pat dry with paper towels; discard the peppercorns. In a heatproof bowl, soak the porcini in the boiling water for 15 minutes; drain in a sieve and rinse under cold running water.
In a large casserole, cook the bacon over moderate heat, stirring often, until lightly browned, about 7 minutes. Using a slotted spoon, transfer the bacon to a small bowl. Season the chicken with salt and pepper. Add the chicken to the casserole in batches and cook until browned all over. Transfer the chicken to a platter. Add the reserved vegetables to the casserole and cook, stirring, until lightly browned, about 5 minutes.
Return the chicken to the casserole and sprinkle it with 1/4 cup of the flour. Bake the chicken for 10 minutes, or until the flour has browned. Add the stock, the reserved marinade and the porcini to the casserole, cover and bring to a boil. Reduce the oven temperature to 300° and bake the chicken for about 50 minutes, or until cooked through.
Meanwhile, blanch the pearl onions in boiling water for 1 minute, then drain. Peel the onions. In a saucepan, combine the onions with 1 1/2 tablespoons of the butter, the sugar and 1/2 teaspoon of salt. Add water just to cover and boil over moderately high heat, shaking the pan, until the water has evaporated and the onions are lightly caramelized, 10 to 15 minutes.
In a large skillet, melt 1 1/2 tablespoons of the butter. Add the white mushrooms, season with salt and pepper and cook over moderately high heat, stirring often, until lightly browned, about 10 minutes.
Transfer the chicken to a platter, cover and keep warm in a low oven. Strain the cooking liquid into a medium saucepan and bring to a boil. In a small bowl, blend the remaining 2 tablespoons of butter and 2 tablespoons of flour to a paste. Skim the cooking liquid, then gradually whisk in the paste until the sauce is thick enough to coat the back of a spoon. Whisk in the tomato paste and season with salt and pepper.
Stir the bacon, white mushrooms and pearl onions into the sauce and bring to a boil. Spoon the sauce over the chicken and serve with the spaetzle.