Prepare the Salmon: Pat the salmon fillets dry with kitchen paper. Season both sides generously with salt and pepper. This step is crucial for a good sear. The salmon should feel dry to the touch, not slippery.

Sear the Salmon: Heat the olive oil in a large, heavy-based skillet over medium-high heat. When the oil shimmers and a drop of water sizzles instantly, carefully place the salmon fillets skin-side down (if using skin). Sear for 4-5 minutes without moving them. You'll hear a steady sizzle, and the edges will turn a deep golden brown. The skin should be crispy and release easily from the pan. Flip and cook for another 2 minutes on the other side. The flesh will look opaque and feel firm to the touch. Transfer the salmon to a plate and set aside.

Sauté the Aromatics: Reduce the heat to medium. In the same skillet, add the diced onion. Cook for 3-4 minutes, stirring occasionally, until it becomes translucent and soft, with a faint sweet aroma. Add the minced garlic and cook for a further 30 seconds until fragrant, being careful not to let it burn.

Build the Sauce: Stir in the chopped sun-dried tomatoes and dried oregano. Cook for 1 minute, allowing the tomatoes to release their flavour. You'll notice the pan looks quite dry at this point. Pour in the chicken stock and bring it to a gentle simmer, scraping up any browned bits from the bottom of the pan with a wooden spoon. The liquid will reduce slightly and become more concentrated in flavour.

Make it Creamy: Reduce the heat to low and pour in the double cream. Stir gently until the sauce is smooth and fully combined. The colour will change from a clear broth to a pale, creamy gold. Add the grated Parmesan cheese and stir until it melts completely into the sauce. The sauce should now be thick enough to coat the back of a spoon, with a silky, glossy appearance.

Wilt the Spinach: Add the fresh spinach to the skillet in handfuls. Stir gently as it wilts. The leaves will turn a vibrant, dark green and reduce significantly in volume. This takes only about 1-2 minutes. Taste the sauce and adjust seasoning with salt, pepper, and red pepper flakes if desired.

Finish the Dish: Return the seared salmon fillets to the skillet, nestling them into the creamy sauce. Spoon some of the sauce over the top of each fillet. Let everything simmer together on low heat for 3-4 minutes. The salmon will finish cooking through, and the sauce will thicken slightly more. The fish should flake easily when tested with a fork, and the sauce will have a luscious, velvety texture.

Rest and Serve: Remove the skillet from the heat. Let it rest for 2-3 minutes. This allows the salmon to relax and the sauce to settle. Garnish with fresh basil or parsley and serve immediately.