Cook pasta as directed on package. Drain. Set aside.
Melt butter in a large skillet over medium heat.
Add mushrooms and onions. Cook, stirring occasionally, 8 minutes or until lightly browned and soft.
Add garlic. Cook, stirring, 1 minute until fragrant.
Add ale, white sauce mixture and tomatoes. Bring to a boil and stir constantly until sauce thickens.
Add shrimp. Reduce heat to medium low and simmer 2- 3 minutes, until cooked through.
Place pasta in a large serving bowl.
Add shrimp and sauce and toss to mix and coat.
When serving, sprinkle with the scallions and Parmesan cheese.