Preheat oven to 375°F (190°C). Pat shrimp completely dry with paper towels.

In a large skillet over medium heat, melt 4 tablespoons of butter. Add onion and cook for 5-6 minutes until soft. Add garlic and cook for 1 more minute until fragrant.

Sprinkle flour over the onions and stir constantly for 1 minute.

Slowly whisk in the half-and-half until smooth. Bring to a simmer and cook until the sauce thickens, about 2-3 minutes.

Remove from heat. Stir in mayonnaise, Worcestershire sauce, and lemon juice until smooth.

Gently fold in the raw shrimp, parsley, salt, and pepper. Pour this mixture into a 9-inch pie plate or similar casserole dish.

In a small bowl, combine the crushed crackers, 3 tablespoons of melted butter, and Old Bay seasoning (if using). Toss to combine.

Sprinkle the cracker topping evenly over the shrimp filling.

Bake for 20-25 minutes, until the filling is bubbly and the topping is golden brown.

Let the pie rest for 5-10 minutes before serving to allow the sauce to set. Garnish with more fresh parsley if desired.