In a medium skillet, heat 6 tablespoons of the oil over medium heat. Add the celery and 4 of the garlic cloves and cook until the celery is wilted, about 3 minutes. Add the tomatoes, parsley and salt and black pepper to taste and cook until the sauce is lightly thickened, about 10 minutes. Transfer Meanwhile, in a large skillet, heat 2 tablespoons of the remaining oil over medium heat. Add the remaining 2 cloves of garlic and cook until golden brown, about 3 minutes. Add about half the spinach and the peperoncino and cook, tossing, until the spinach is wilted. Add the remaining spinach a handful at a time as the spinach in the skillet wilts. Cover the skillet and cook, stirring occasionally, until the spinach is tender, but still bright green. The whole process should take about 5 minutes. Keep warm. Pat the halibut fillets dry with paper towels and season them generously with salt and pepper and coat them with the remaining oil. Heat a large skillet, preferably non-stick, over medium-high heat. Add the halibut and cook, turning once, until well browned and barely opaque in the center, about 8 minutes. Drain any oil from the pan and keep the halibut warm. Drain any liquid from the spinach and divide the spinach among 6 serving plates. Top with a piece of halibut. Remove the garlic cloves from the tomato sauce and spoon some of the sauce over and around the fish. Serve immediately.