Heat oven to 200*F. Heat 1 TBS olive oil in a skillet over medium heat. Add onion, 1/2 tsp salt, and 1/8 tsp pepper. Cook until the onions are translucent, about 5 minutes. Set aside.
Cut fish into large chunks; pulse in a food processor to coarsely chop. Transfer to a medium bowl. Add onion, tarragon, egg and Tabasco, combining well. Add remaining salt and pepper. Form 8 3-inch patties; dredge them in bread crumbs, shaking off excess.
Heat 1 TBS oil in a large skillet, over medium low heat. Cook 4 patties until browned, about 4 to 5 minutes on each side. Remove to a baking sheet, cover with aluminium foil, and keep warm in oven while you cook the other 4. Wipe out skillet, return to heat and add remaining oil. Cook remaining patties and serve immediately with tartar sauce.