Aromatics, sliced or chopped, to taste (garlic, onion, shallots, ginger, lemongrass, chilies, bacon, salami, you name it. Just make sure it's tasty stuff.)
½ cup wine, beer, juice, or whatever liquid you'd like (use more for a brothier dish, but the mussels themselves will release a lot of juice)
2 pounds mussels, cleaned (see above)
Herbs, chopped (parsley, thyme, rosemary or others) or other delicate flavor additions, to taste (orange zest? A little more raw shallot?)
Butter, cream, olive oil, ground nuts or other finishing touch to enrich the broth, to taste
Lemon, vinegar or some kind of tart flavoring, to taste, if your liquid isn't very bright
Salt and pepper, to taste (mussels do tend to be salty, so this might not be necessary)
Grab a pan big enough to fit all the mussels comfortably, preferably with a lid. Get it hot over medium heat. Add a touch of butter or oil, and sweat or sauté your aromatics. When they're throwing off delicious smells, add the liquid and turn the heat up to high.
When the liquid is boiling, add the mussels all at once, cover the pan, and give it a couple of good, hard shakes. Peek under the lid after about two minutes to see how they're doing. Once they're open, they're cooked. Give the pan another shake, and another after two minutes or so, until all the shells are open. (If there are stubborn stragglers, way behind the rest, just ditch them. They might be dead, and you don't want to overcook the rest of the mussels waiting for the dead to make contact.)
Now have a taste of the broth. Season it with salt and pepper if need be, but here's a tip -- when you season, tip the pan and season directly into the broth, and stir it in to dissolve. (Just tossing salt into the pan might get a bunch of it tucked into the mussels' shells, and you won't be able to really tell how seasoned the broth is.)
Add your herbs, butter and/or other finishers. Stir or toss to combine everything and emulsify the butter to a creamy sauce, and serve right away.
Francis Lam is Features Editor at Gilt Taste, provides color commentary for the Cooking Channel show Food(ography), and tweets at @francis_lam.