Melt butter in a skillet over medium heat, then cook onion and garlic until soft and fragrant, about 3–4 minutes. Stir in flour and cook for a minute to form a roux. Gradually whisk in heavy cream and sour cream until smooth, then add cheddar cheese, Old Bay seasoning, salt, and pepper.
Once the cheese melts, stir in seafood and cook for 2–3 minutes until opaque. Add lemon juice and parsley, then remove from heat.
In a large bowl, mix flour, baking powder, salt, and cheddar cheese. Cut in cold butter until coarse crumbs form, then add milk a tablespoon at a time until the dough comes together. Roll or pat it to fit your pie dish, adding a dusting of flour if sticky.
Preheat oven to 375°F (190°C). Pour filling into a greased dish, top with crust, and lightly seal edges. Cut small slits for steam, then bake 25–30 minutes until golden and bubbly.
Let rest for 5 minutes, garnish with parsley, and finish with a squeeze of lemon before serving.