These are relatively inexpensive, very classy, delicious and good diet food. Low carb, high protein and moderate cals. And easy!!
Buy 2 lbs or so of fresh, live mussels. Cultivated mussels have been grown in the ocean on floating nets, so not much sand and grit inside them. Dump the mussels in a sink of cool water and sort them. Any that don't hold themselves tightly closed are dead, discard them. If one is hanging a bit open, bonk it against the side of the sink. If it closes up, it's OK, otherwise discard. Some mussels have a few hairy tentacles hanging outside the shell along one side. This is called the beard, and you should pull them out. This is harder than it sounds - they hold on tight! Set aside your cleaned mussels for a few minutes.
In a big pot with a good lid, melt 2-3tbsp butter. Add and saute a chopped onion, 1-2 stalks of chopped celery and a very thinly sliced or jullienned carrot (or you can use shallots to be really authentic - I'm not!) Once they're nice and soft, add in about 1-2 cups of dry white wine (then pour yourself a glass of the remainder if you like), dump in the mussels, and put the lid on. Crank the heat up to med-high and don't lift the lid for 5-8 min.
When it's done, all the mussels will be opened and ready to be eaten out of the shell. Serve onto a shallow bowl (pasta bowls work great) and be sure to scoop the broth, too. The broth is traditionally meant to be soaked up with crusty white bread, but you can just eat it with a spoon if you prefer.
Put an extra plate on the table for the shells.