Scrub the mussels, and yank out any beards slipping through the shell with a pair of pliers. Put them on a bunch of ice cubes in a bowl and refrigerate until you need them. I’ve kept them that way for 1 day.
Melt the butter in a large pot over medium-high heat – I use my Le Creuset large Dutch oven. Add the olive oil and heat it up. Add the onion and pepper (it should sizzle when it hits the pan) and sauté until softened, about 6 – 7 minutes. Add the garlic, salt and pepper and cook for another couple of minutes.
Thoroughly stir in the curry paste and cook for 5 minutes. Add the coconut milk, vegetable stock, lime juice, and stir. Bring to a simmer, stirring continuously.
Reduce the heat to medium and carefully add the mussels. If you pile them in with abandon, the shells may break. Cover the pot and simmer just until the mussels open, 5 – 6 minutes. If you forget about them, they'll get tough and rubbery, which no one likes.
Ladle some broth into bowls, followed by the mussels, and finishing with more broth. Be sure to discard any shells that don’t open. Garnish with sliced scallions or cilantro if you choose. Serve immediately with sliced, warm baguettes with butter.
Calories were calculated using the VeryWell Fit nutrition calculator, and assumed 2 servings.