Bring the salmon fillet up to room temperature. This will help to ensure an even cook.
Warm the oil in a frying pan over medium heat and add the salmon fillet, skin side down. Season to taste and sprinkle in the fresh thyme and the lemon juice, if using. Cooking time will depend entirely on the thickness of the fillet, however as a rough guide, a salmon fillet 5cm thick will need about 8 to 10 minutes skin side down, a few minutes each side and then finally a minute or so for the top to get a nice golden blush. The salmon is done when it is evenly pink throughout and flakes easily with a fork.
Remove the fillet from the pan and allow to rest on a warm plate for a few minutes before serving.
Cooking a salmon fillet takes a bit of practice and if overcooked it can become tough and dry. My advice is to under-cook it slightly, then let it rest for a few minutes to continue cooking in the middle. It is ready when the salmon is evenly pink throughout, juicy in the middle and flakes easily with a fork.