Preheat oven to 450°F.
Stir broth, cream, garlic powder, ¼ teaspoon salt and ⅛ teaspoon pepper together in a large skillet. Add mushrooms and orzo, cover and bring to a boil over high heat. Stir in spinach, reduce heat to maintain a lively simmer, cover and cook until the orzo is tender and the sauce has reduced and thickened, 10 to 12 minutes.
Meanwhile, brush oil on both sides of salmon pieces. Sprinkle with the remaining ¼ teaspoon each salt and pepper. Place on a baking sheet and roast until just cooked through, 8 to 10 minutes. Serve the salmon with the orzo.