Keep the mussels refrigerated until you're ready to use. Once ready, place them in a large bowl of ice cold water. Scrub the outsides of the mussels and remove the string (or "beard") by using a towel or paper towel to pull it out. Discard any mussels that have opened already. Keep the mussels in the ice water.
Heat a large skillet or wok over medium heat and add the butter and olive oil. Once it's melted, stir in the onions, garlic, salt and pepper. Stir and cook for 5 minutes, until slightly softened. Stir in the curry paste and continue to stir until it's distributed throughout the entire pan of onions. Cook for 5 more minutes.
Stir in the coconut milk and stock, stirring until it's smooth. Bring the mixture to a simmer, stirring continuously. Reduce the heat to medium low.
Add the mussels and toss. Cover the skillet and cook just until the mussels open, about 5 to 6 minutes. Garnish with the sliced green onions. Stir the mussels well so the broth makes it into the shells. Serve immediately with toasted baguettes