For the rub: Put the fennel seeds in a small bowl and crush lightly with the back of a wooden spoon. Add the dried onion, cayenne, cumin, oregano, paprika, salt and pepper and mix together.
For the salmon: Spoon the rub onto the salmon fillets on both sides and rub in. (Don't do this more than 5 minutes ahead of cooking or it will start to affect the fish and soften it.)
Heat a large pan over high heat, add the olive oil and heat until very hot. Add the salmon fillets skin-side down. Cook on high until the flesh turns opaque about halfway up the side of the fillets, about 5 minutes. Cover with a lid or an inverted frying pan. Turn the heat off and leave it for 5 minutes; the heat and steam will cook the top of the fish so it will be crisp on the bottom and cooked through.
For the salad: Meanwhile, add the arugula and spinach to a bowl. Add the red onion to the center and scatter the fennel and fronds around the edges of the bowl. Drizzle the balsamic only on the red onion. Sprinkle the rest of the salad with salt and pepper and drizzle with the olive oil. Toss everything together.
Add a heap of salad to 2 plates and top each with a salmon fillet.
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