Bring three quarts water to a boil in a large saucepan. Add the beans and cook until tender enough to be split with a fork, 8-10 minutes (cooking time will vary depending on the age of the beans). Drain. Set aside.

In the meantime, pour oil into a 10- to 12-inch skillet over medium heat. Add onion and cook until translucent, about five minutes. Stir in the fresh mushrooms and cook for 2-3 minutes more. Set aside.

Place dried mushrooms in a 2-cup glass measure with 1/2 cup water. Cover tightly with plastic wrap and microwave on high for three minutes. Drain.

Cut the dried mushrooms into 1/4-inch pieces with scissors. Add with the snap peas to the onion-mushroom mixture and cook over medium heat for 3-4 minutes, stirring occasionally. 

When the snap peas are almost cooked through, add the cooked string beans. Season to taste with salt and pepper.

Toss with pasta and serve warm.