The flavor of walnuts can vary depending on their freshness and country of origin. If you have bought some which you find bitter, their flavor can be mellowed by blanching them. Crack open the nuts and put the kernels in a small saucepan of boiling water. Boil gently for a minute, then drain and dry before using.
For the best flavor, make sure that walnuts are absolutely fresh. To test the freshness of shell-on nuts, weigh them in your hand individually - each one should feel heavy and not rattle when shaken. If the nut rattles, the kernel inside will have dried and shriveled. If, after blanching, you still find the flavor too bitter, you could use mellower pecan nuts instead, which are from the same family.
** Asian Recipes