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.
This is my favourite way to cook salmon, and the addition of thyme worked really well. I'll definitely try this again! - 21 May 2015