Preheat oven to 375°. In a small bowl whisk together 1 tbsp extra virgin olive oil, ginger, and zest and juice of half the lemon. Place salmon on a plate, pat dry, and sprinkle with salt and pepper. Brush mixture generously onto salmon and set aside.
Place mushrooms in a medium bowl and toss together with 1 tbsp olive oil, garlic, oregano, and a heavy pinch of salt and pepper. Set aside.
Heat 1 cup water over high heat in a covered medium-sized saucepan. When water is boiling stir in couscous, butter, and a pinch of salt. Cover and reduce heat to low. Simmer, stirring occasionally, for about 10 minutes until al dente. Remove heat and leave covered.
While couscous is cooking, spread mushrooms onto a large sheet pan. Bake for 10 minutes. Stir mushrooms and add salmon to the sheet pan. Return to oven for 5-10 minutes until salmon flakes easily with a fork. If needed remove salmon to a plate and continue to roast mushrooms until lightly browned and tender.
Stir arugula and juice from remaining half of lemon into the couscous. Divide into bowls. Flake salmon with a fork into bite-sized chunks and scatter on top of couscous along with mushrooms and capers, if using.