Pop the rainbow trout fillets on a chopping board. Dust with salt and pepper.
Tip the raisins in a mug. Pour 3 tbsp boiling water over them. Set aside to soak.
Strip the rosemary leaves from the woody stalks. Finely chop the leaves. Finely chop the chilli, flicking out the seeds and white bits for less heat. Pop in a bowl. Add 1 tsp fennel seeds. Grate in the zest of the lemon. Press half of the mixture all over your fish, coating both sides.
Halve, peel and thinly slice the onion. Halve your fennel and cut into 1-2cm thick slices.
Set a large frying pan over high heat. Add 1 tsp oil. Arrange the fennel and onions in the pan in a single layer - a little overlapping is okay. Season well. Cook over medium-high heat till the fennel is tender and caramelised underneath, about 10 mins. All the remaining fennel and lemon mix for the final 2 mins of cooking.
Drain the raisins. Tip back into the mug. Add 2 tbsp lemon juice, 1 tsp olive oil and a pinch of salt. Whisk to make a dressing. Place the watercress in a bowl and toss with the dressing.
Set a separate frying pan over high heat. Add 1 tsp oil. Place the fish in the pan, skin side down. Cook 3 mins or till the skin is crisp. Flip and cook for 1 min or till cooked through on the other side - the colour will go from a rich salmony orange to a light, pale pink.
Pile the watercress, fennel and onion on plates. Serve with the crisp fried trout.