Preheat the grill. Line a baking tray with aluminium foil. Place the salmon on the foil and brush with 2 tablespoons melted butter. Season with salt and pepper. Grill 15 minutes, until fish is easily flaked with a fork.
Melt the remaining butter in a frypan over medium heat and saute the mushrooms until tender.
Place the tomatoes in a bowl and drizzle with 1 tablespoon olive oil. Season with salt and pepper.
In a large bowl, toss together the salmon, mushrooms, tomatoes, lettuce, avocado, coriander and chilli. Drizzle with remaining olive oil and the vinegar. Season with salt and pepper and sprinkle with feta cheese to serve.