Pasta shells with mussels and peas

3/4 pound large pasta shells
3 tablespoons olive oil, or vegetable oil
2 medium onions, peeled and chopped
2 cloves garlic, chopped
1/4 pound mushrooms, quartered
1/4 cup dry vermouth
3 cups mussels, steamed,shelled
1 1/2 cups mussel cooking broth
or 3/4 c clam broth and 3/4 c chicken broth
4 tablespoons fresh basil, chopped
or 2 teaspoons dried basil
2 cups fresh peas, blanched 1 min
Fresh ground black pepper
2 to 3 Tb parsley, chopped

Put a large pot of salted water on to boil, and when it comes to a rolling boil, drop the pasta shells in. Meanwhile, heat the oil in a skillet and saute the onions slowly for about 5 minutes, until they are almost tender, then add the garlic and the mushrooms and saute another couple of minutes, stirring. Add the vermouth and cook rapidly to reduce by half, then stir in the mussels, broth, and basil, and bring to a simmer. Drain the pasta when it is done al dente, and mix with the sauce, folding in the peas, and heating through.
Season with freshly ground pepper (it probably will not need salt). Serve in warm bowls with parsley sprinkled on top.