Melt the butter in a large skillet over medium heat. Add the onions and thyme and cook until the onions are softened, about 3 minutes. Add the chicken livers and cook until firm but still pink inside, about 3 minutes.
Remove the skillet from heat, add the salt, pepper and brandy, and deglaze the pan by stirring and scraping up the brown bits from the bottom; cool.
Remove the thyme stems from the skillet, transfer the chicken liver mixture to a blender, puree until smooth and transfer to a bowl.
In another medium bowl, use an electric hand mixer to whip the heavy cream to medium peaks. Fold the whipped cream into the liver mixture, taste and adjust the seasoning. Cover with plastic wrap and chill for 30 minutes.
Serve the mousse with toasted baguette slices and garnish with pickled onions.