Preheat oven to 400°F (200°C). Melt 6 tbsp of butter in a large Dutch oven over medium heat. Add diced onion, carrots, and celery and sauté for 8-10 minutes until softened. Stir in minced garlic and cook for 1 minute until fragrant.

Sprinkle the 1/2 cup of flour over the vegetables and stir constantly for 2 minutes to cook out the raw flour taste.

Slowly whisk in the milk until smooth, then whisk in the heavy cream, Old Bay, salt, and pepper. Bring to a simmer and cook for 5 minutes until thickened. Remove from heat.

Gently stir the raw shrimp, lump crab meat, and frozen peas into the hot cream sauce. Pour the filling into a 9x13 inch (or equivalent 3-quart) baking dish.

In a large bowl, whisk together 2 1/2 cups flour, baking powder, and salt. Add the shredded cheddar. Cut in the cold, cubed butter until the mixture resembles coarse crumbs. Stir in the cold buttermilk until just combined; do not overmix.

Drop large spoonfuls of biscuit dough over the seafood filling, covering the surface. Bake for 25-30 minutes, or until the filling is bubbly and the biscuit topping is deep golden-brown.

Remove from the oven and immediately brush the hot biscuits with 2 tbsp of melted butter. Sprinkle generously with fresh parsley. Let the pot pie rest for 10 minutes before serving.