Thaw mussels and rinse well in deep cold water. Drain and rinse in deep water again, in case there are any hiding bits of shell or grit in there.
Cook the spaghetti according to package directions and drain.
Heat the oil in a large skillet, adding shallots and garlic.
Stir in the mushrooms and pimientos and cook until the mushrooms soften.
Add the scallions, parsley and basil, then deglaze with the white wine, sweet vermouth and lemon juice.
Add the mussels to the skillet then stir in the cream and milk. Heat until steaming, then pull from the heat and stir in the Parmesan, allowing it to gently melt.