Toasted baguette slices and Spiced Prune Chutney, for serving
In a medium bowl, toss the chicken livers with the brandy, bay leaves and crushed garlic; let marinate at room temperature for 2 hours or refrigerate for up to 8 hours. Drain the livers and pat dry; discard the bay leaves and garlic.
In a large, deep skillet, heat the oil. Add the onion, pancetta, anchovy and sliced garlic and cook over moderately high heat, stirring, until the onion and garlic are softened, 8 minutes. Add the tomato paste and stir for 1 minute. Stir in the chicken livers. Add the vinegar and cook until nearly evaporated. Add the vermouth and cook until nearly evaporated and the livers are just pink within, about 3 minutes. Scrape the mixture into a food processor; let cool slightly.
Add the cream, thyme and sage to the skillet and cook over moderately high heat, stirring, until the cream is reduced to 1/2 cup, about 8 minutes. Pour into the food processor and puree. Press the mousse through a fine sieve set over a bowl; season with salt and pepper. Press plastic wrap directly onto the surface of the mousse and refrigerate until chilled. Serve the mousse with toasted baguette slices and the Spiced Prune Chutney.