Preparation

• Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 3-quart casserole dish.
• Fill a large pot with water, and bring to a boil over high heat. Stir in the shrimp, reduce heat to a simmer, and cook the shrimp until opaque and bright pink, about 3 minutes. Remove the shrimp to a bowl with a slotted spoon. Return the water to a full rolling boil over high heat, and stir in the noodles. Cook until the noodles are tender, about 8 minutes; drain in a colander set in the sink.
• Melt 2 tablespoons of butter in a saucepan over medium heat, and stir the flour into the butter to make a paste. Remove the pan from the heat; slowly whisk in the milk and cream until smooth. Season to taste with salt and black pepper. Return the sauce to the heat, lower heat to a simmer, and whisk constantly until the sauce thickens, about 5 minutes.
• Place noodles into the prepared casserole dish, and top with peas, mushrooms, and cooked shrimp. Pour the sauce over the casserole, and sprinkle the crushed cracker crumbs evenly over the top. Dot the top of the casserole with about 6 thin slices of butter.
• Bake in the preheated oven until the cracker topping is crisp and golden brown, about 1 hour.