The secret: We are going to cut the mussels open raw , this is what makes the taste of this recipe. it will take a few mussels to take the hang of it but you'll manage just fine i'm sure.
Use a sharp potato knife , take the mussel between your thumb and index and nudge it open a bit with a sliding movement ( should need some force) slip the knife in between and scrape the side of the mussel until the big tendon is cut (cut one open any way you want so you can see where the big tendon is located it's in the same place for each mussel), now you can open the mussel a bit and use the knife to make sure the meat is all on 1 side . tear off the empty side of the mussel and place the full side on a big oven dish (the metal thing in your oven) , if you can't get the mussels to stay horizontal , you can use a bit of flour and water to make a dough and put a small ball of this under each mussel , to keep it level , but this isn't surgery so about level without the dough is more than ok.
While you do the mussels , your other half can make the herb butter . Make sure your butter is about room temperature , mix in the parsley , the garlic and the shallots. also add a pinch of salt and some pepper here. (although salt can be omitted since the mussels contain some seawater still , but i use a bit of salt). Stir the whole mixture up , and let it cool in the fridge a bit so the butter is a bit harder and easier to use later on. now go help your partner cut open the mussels :).
Preheat your oven at 400°F.
When all the mussels are open and face up on the tray put about half a teaspoon of your herb butter on each mussel . Gently put the tray ion the oven . cooking should take only a few minutes (keep watching it ), they are done when they've all bubbled for a minute and the onions/shallots start getting some color (they should look golden brown at a glance ).
Take the whole tray out and place on a fireproof holder in the middle of the table , let people pick them up one by one as they eat . Don't forget to use a piece of baguette to dip in the juice thats in each muscle , that's the best part :).