In a large skillet, melt 2 tablespoons of the butter in the olive oil until foaming. Add the chicken livers and cook over high heat, turning once or twice, until browned all over, about 3 minutes. Transfer the livers to a small bowl.

Add the bacon to the skillet and cook over moderate heat, stirring, until browned and crisp, about 5 minutes. Using a slotted spoon, transfer the bacon to a paper towel-lined plate to drain.

Add the onion to the skillet and cook over moderate heat, stirring frequently, until softened and lightly browned, about 10 minutes. Add the apple, reduce the heat to low and cook, stirring, until very soft and just beginning to break down, about 10 minutes longer. Add the brandy to the skillet and carefully ignite with a long match; let the alcohol burn off.

Add 1/4 cup of the apple mixture to the bacon. Add the remaining apple mixture to a blender along with the chicken livers and blend until smooth. With the machine on, add the remaining 4 tablespoons of butter and puree until creamy. Pass the chicken liver mousse through a fine strainer into a bowl (omit this step for a chunkier mousse). Season with salt and pepper and refrigerate until firm, at least 30 minutes.

Spread the chicken liver mousse on the baguette toasts. Garnish with a little of the bacon-apple mixture and the sage leaves and serve at once.

The mousse and apple-bacon garnish can be refrigerated separately for up to 2 days. Return to room temperature before serving.