Whisk together white sauce mix and milk in a bowl.
Melt butter in a large skillet over medium heat. Add mushrooms and onions and 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 to 3 minutes until cooked through.
Drain pasta and place in a large serving bowl. Add shrimp and sauce; toss to mix and coat. When serving, sprinkle with the scallions and cheese.