Preheat the oven to 400°F (200°C).
Grease a large baking dish with olive oil. Place the salmon, with the skin-side down, in the prepared baking dish. Generously season with salt and pepper.
Slice the lemon thinly and place on top of the salmon. Cover with half of the butter in thin slices.
Bake on the middle rack for about 20–30 minutes, or until the salmon is opaque and flakes easily with a fork.
Heat the rest of the butter in a small saucepan until it starts to bubble. Remove from heat and let cool a little. Gently add some lemon juice.
Serve the fish with the lemon butter and a side dish of your choice. See below for suggestions.