With a thin, sharp knife, carefully cut off the fleshy portions of each halibut steak, or halibut tail piece. Trim the skin of those pieces, and then cut the flesh into cubes and store in the fridge until needed. Place the halibut bones and skin in a small pot. Pour in the stock, simmer for 20 minutes, and then strain. Melt the butter in a medium-sized pot set over medium heat. Add the onion, carrot, celery and garlic and cook 4 to 5 minutes. Stir in the flour and tarragon until well-combined. While stirring, slowly mix in the stock. Bring to a simmer and cook until the vegetables are tender, about 5 minutes. Add the cubed halibut and frozen vegetables and simmer 4 to 5 minutes more, or until the halibut is just cooked through. Season with salt and pepper and serve.