Preheat the oven to 425 degrees F. Line a shallow baking dish with parchment. Set in the fish in a single layer. Combine 1 Tbsp. of the olive oil, garlic, lemon juice and 1 tsp. of the thyme in a small bowl. Brush on the salmon; season with salt and pepper. Bake the salmon 12-15 minutes, or until cooked through. While the fish cooks, heat the remaining oil in a skillet over medium heat. Add the leek and mushrooms and cook until tender, about 5 minutes. Add the remaining thyme and the stock; gentle simmer until the stock is reduced by half. Divide the leek and mushroom mixture among plates. Set on the salmon and serve.