Melt the butter in a large saute pan and spread the shallots over the bottom. Sprinkle with salt. Turn the heat to low and cook for 30 minutes, stirring occasionally.
Pull the leaves from the thyme and stir them into the carmelized shallots. Remove from pan and set aside in a bowl.
Place the pan back on the stove, add the vinegar and turn heat to medium high. Stir to deglaze the pan, and pour sauce over shallots. Add the creme de cassis, a little at a time, to taste. Allow the shallots to cool, then stir in the currants or currant jam. If you are not serving dinner in the next hour, refrigerate.