Heat 2 tablespoons (30 ml) of the butter in a skillet over moderate heat and saute the onion until tender but not browned, about 5 minutes.
Add the garlic and cook 1 minute.
Add the chicken livers, thyme, salt, and pepper and saute until the chicken livers turn color-they should still be slightly pink in the centres.
Add the optional brandy and cook 1 minute. Transfer the contents of the skillet to an electric food processor and process, adding the remaining butter a little at a time and scraping down the sides of the processor once or twice, until smooth.
Press through a fine sieve and pack into a 2-cup (500 ml) loaf pan that has been lined with plastic wrap.
Chill for at least 3 hours to overnight.
To serve, dip the pan in warm water and invert onto a cutting board.
Remove the plastic wrap and smooth with a knife.
Serve with toast points, crackers, or Melba toast.