We were introduced to mussels in a bucket in Datca in 1990. Best street food ever. We make a point of hunting down the Mussel man in Calis and they are excellent. You can buy one or the whole bucket. Its up to you. Its traditional to stand by the guy whilst you eat,until your full, then pay up.
We did ask one guy how you stuff them. Not sure if this translation is right but he said "You tap the closed shell with a spoon. When the mussel sticks its head out, you shove the rice in quick before he realises he is about to be had"