Season the salmon filets with the lemon zest, freshly ground pepper, and salt. Grill over moderate heat for about 4 minutes per side. Keep warm. Meanwhile, in a large sauté pan, melt the butter over medium high heat. Add the mushrooms, parsley, thyme and garlic and sauté for 4 minutes, stirring frequently. Add the apples and sauté for three minutes more. Stir in the verjus and white wine and cook for about 4-5 minutes, or until reduced slightly. Season to taste.
To serve, divide the mushroom-apple mixture amongst the plates. Arrange the fish on top and garnish with thyme spring.