Defrost the mussels if necessary. Peel and finely dice the shallots. Wash and drain the salad. Mix vinegar, herbs, half shallots, some salt, pepper and about 1 pinch of sugar. Fold in 3 tablespoons of oil.
Wash the mussels and pat them dry well. Heat 2 tablespoons of oil in a frying pan. Fry the mussels on each side for about 1 1/2 minutes. Season with salt and pepper and remove.
Cut butter into cubes. Steam remaining shallots in hot frying fat until translucent. Add crémant, bring to the boil and simmer for about 1 minute. Gradually add butter over a low heat.
Season with salt, pepper and 1 pinch of sugar. Heat the mussels briefly in the cream sauce.
Mix salad and vinaigrette and serve with the scallops. Spread the sauce on the scallops and serve immediately.