For the biscuits: Preheat the oven to 375 degrees F.
Whisk together the flour, baking powder, sugar, garlic powder and kosher salt in a large mixing bowl. Using a pastry cutter or 2 butter knives, cut the butter into the flour mixture until pea-sized (or pulse in a food processor). Stir in the cheese and tarragon. Make a well in the center and stir in the buttermilk until just combined.
For the filling: Heat the butter in a large skillet over medium heat. Add the carrots and onions and cook until the onions are translucent and the carrots are tender, about 8 minutes. Stir in the peas and corn and cook for 1 to 2 minutes. Stir in the flour and cook about 2 minutes. Stir in the stock, clam juice, sherry, salt, pepper and lemon zest and juice. Bring to a low simmer and simmer until the mixture thickens enough to coat the back of a spoon, about 5 minutes. Stir in the lobster, heavy cream and hot sauce. Transfer the mixture to a 9-by-13-inch casserole dish.
Drop the biscuit dough by heaping tablespoons on top of the lobster mixture. Sprinkle with flaky sea salt. Bake until the casserole is bubbly and the biscuits are golden brown, about 40 minutes.