Melt 60 g of butter in a frying pan over low heat. Sauté the onion, garlic, thym and rosemary for 6 to 8 minutes, stirring, until the onion is very tender. Add the livers and cook over medium heat, stirring, until they are well browned on the outside but still pink in the center. Remove from heat and let cool. Blend the livers, remaining butter, salt and pepper in a blender to obtain a smooth paste. Using a piping bag fitted with a small star-shaped nozzle, spread this preparation over the crackers. Decorate with olive slices and serve immediately.