Cover mushrooms with hot water in a bowl. Let stand 10 minutes. Drain, rinse and cut into 1/2 inch pieces. Melt butter in a heavy nonstick skillet over medium heat. Saute shallots 1 minute. Add mushrooms and tomatoes and continue to saute 3 minutes, stirring frequently. Stir in milk, salt, and white pepper. Bring to a boil. Reduce heat to low and simmer 10-15 minutes, or until volume is reduced by 1/4. Meanwhile, cook fettuccine in boiling water until al dente. Drain and transfer to a large bowl. Pour sauce over pasta and toss. Sprinkle with Parmesan and scallions.