Pat the salmon fillets completely dry with paper towels and season both sides generously with salt and pepper.
Heat the olive oil in a large non-stick skillet over medium-high heat until it begins to shimmer.
Place salmon in the pan, skin-side down, and press gently with a spatula for 10 seconds to keep the skin flat.
Cook undisturbed for 4–5 minutes until the skin is crispy and releases easily from the pan.⏱ 5 min
Flip the fillets carefully and cook for another 2–3 minutes until the salmon is opaque and flakes easily with a fork.⏱ 3 min
Remove the salmon from the pan and set it aside on a plate.
Add the butter to the same pan and let it melt until it foams, then toss in the spinach and cook for 1 minute until wilted.⏱ 1 min
Stir in the lemon juice, season with salt, and serve the spinach alongside the salmon with the lemon slices on top.