Slice two thin circles in the centre of the lemon, then halve both of them to create 4 half moons (to serve). Use the remaining lemon to squeeze 1 1/2 tbsp of juice.
Pat the salmon fillets dry with paper towels/kitchen roll, then season both sides with the salt, pepper and garlic powder.
Place a large non-stick pan over medium-high heat and add the oil. Melt in the butter, then once hot, add the salmon presentation side down. Fry both sides for around 3 minutes, or until they build up a golden crust and the salmon is almost cooked through the centre, then remove and place on a plate to one side.
Turn the heat down to medium and stir in the stock, honey and lemon juice. Add the cold butter and swiftly stir until it blends into the sauce. Keep stirring and simmering until the sauce reduces and thickens to a glossy consistency. If the sauce looks like it's splitting at all, just lower the heat and keep swiftly stirring. The sauce will be light, but thick enough to spoon over the salmon and not watery at all.
Stir in the resting juices from the salmon and simmer to re-thicken if needed, then season to taste with salt (generously if needed). Add the salmon to the pan, place the lemon slices on top and baste in the sauce.
Serve the salmon with the sauce drizzled over the top, then garnish with chives and more black pepper.