I've made my first attempt at cooking moules frites however I'm a bit disappointed with the results.
The batch of mussels were purchased from an online fishmonger specialising in Cornish seafood in the UK. Two thirds of the batch had already opened so following advice online I discarded them. The remaining ones I thoroughly washed in cold running water and removed any beards visible. On cooking, they all opened however they all tasted gritty and in the shells I could see black sludge.
Have I done something wrong? Is there an additional preparation step I missed? Or is it a bad batch?
Any help or advice is much appreciated. I haven't cooked fresh mussels before so it's a bit of a learning experience for me!