Zest and juice the orange. Pop the zest and juice in a bowl. Peel and thinly slice the onion.
Pick any feathery fronds off the fennel and keep them for later. Finely slice it. Add the sliced fennel and onion to the bowl with the orange zest and juice. Sprinkle in a pinch of salt and toss together. Set the fennel and onion salad aside.
Bring a pan of water to a rolling boil. Add a pinch of salt. Tip in the pasta. Cook for 8-10 mins till al dente (soft with a slight bite).
While the pasta is cooking, rub each trout fillet with 1 tsp oil. Season. Warm a frying pan over a medium heat. Place the fillets into the pan skin-side down. Fry for 2 mins, then flip the fish over and cook for a further 2 mins. Place on a plate.
Roughly chop the baby leaf spinach. Pick the leaves from the parsley. Finely chop the stalks. Add the parsley stalks to the bowl with the orangey fennel and onion salad.
Peel the skin from the trout. Flake the trout with a fork and set aside.
Drain the pasta, then tip back into the pan. Place over a low heat. Add the baby leaf spinach and stir to wilt it. Pour in the crème fraîche and mix. Add most of the parsley leaves. Add the trout and carefully fold through. Garnish with the remaining parsley and reserved fennel fronds. Serve with the fennel and onion salad.