Place mushrooms in small bowl. Pour boiling water over. Cover and let stand until mushrooms soften, about 15 minutes. Remove mushrooms from liquid; reserve soaking liquid. Coarsely chop mushrooms; discard any tough stems. Combine wine and mushrooms in heavy large skillet. Boil until wine is reduced by half, about 2 minutes. Add cream and tarragon. Slowly pour in mushroom soaking liquid, leaving sediment at bottom of bowl. Boil until sauce is thick enough to coat spoon, about 5 minutes. Mix in green onions and lemon juice. Season with salt and pepper. Meanwhile, cook pasta in large pot of boiling salted water until tender but still firm to bite. Drain pasta. Add pasta to sauce; toss gently to coat and serve.