Preheat your oven to 425°F (220°C).
Heat a drizzle of olive oil in a large skillet over medium heat. Add the shrimp and langostino, season lightly with salt and pepper, and cook for 2-3 minutes until just turning opaque. Remove and set aside.
In the same skillet, sauté the onions, bell pepper, and celery for 4-5 minutes until translucent.
Reduce heat to medium-low. Stir in 4 tbsp of butter until melted. Sprinkle in the flour and stir continuously for 1 minute to create a roux.
Slowly whisk in the chicken broth, then the heavy cream. Bring to a simmer and add garlic, thyme, tarragon, smoked paprika, salt, and pepper. Simmer for 2-3 minutes until thickened.
Gently fold the cooked shrimp, langostino, and crabmeat into the gravy. Remove from heat.
In a mixing bowl, combine the biscuit mix, shredded cheddar, and milk. Stir gently with a fork until a shaggy dough forms. Do not overmix.
Spoon the seafood filling into a large baking dish or 6 individual ramekins. Top with spoonfuls of the biscuit dough.
Bake for 14-16 minutes, or until the biscuit topping is puffed and golden brown.
While the pie is baking, whisk together the melted 1/4 cup of butter and the garlic herb seasoning packet from the biscuit mix.
Immediately brush the hot biscuits with the prepared herb butter sauce. Serve hot.