In a large pot, melt the butter in the oil over medium heat. Add the garlic and onions and season with a pinch of salt and a few grinds of pepper. Sauté until softened, about 5 minutes. Add the yogurt and whisk until smooth. Raise the heat to medium high and slowly whisk in the stock and lemon juice. Bring to a simmer. Immediately add the mussels and cover. Cook until the mussels have opened up, 6 to 8 minutes. Discard any mussels that do not open.
While the mussels are cooking, pile the cilantro, dill, mint, parsley, tarragon, scallions and lemon zest on a cutting board and roughly chop a little more.
Give the mussels a stir, add the herb-zest mixture and stir again. Serve hot with crusty bread to dunk into the cooking liquid.