Cook the ribbon noodles in salted water for 8-10 minutes until al dente. Drain and set aside.
Season the cod fillets with salt and pepper. Heat 1 teaspoon olive oil in a pan over medium-high heat. Fry the fillets for 3-4 minutes per side until the flesh is opaque and flakes easily with a fork. If the fillets have skin, fry skin-side down first.
Dice the onion and sauté in a deep pan with butter over medium heat for 2-3 minutes until softened. Wash the spinach, remove thick stems, and roughly chop. Add the spinach to the pan and stir for 1-2 minutes until wilted. Pour roughly 250 ml (1 cup) of the pasta cooking water into the pan and stir in the 2 tablespoons crème fraîche. Add the capers. Season with turmeric, garam masala, curry powder, salt, and pepper to taste. Simmer over low heat for 5 minutes. Remove from heat and fold in the cooked noodles.
Divide the pasta and sauce among serving plates and top each with a cod fillet.