Cook the quinoa according to the package directions.
Meanwhile, melt the butter in a small pot over medium-high heat. Add the onion and 1/2 teaspoon each salt and pepper. Cook, stirring occasionally, until tender, 6 to 8 minutes. Add the cream and cook until thickened, 2 to 4 minutes. Stir in the spinach and cooked quinoa.
Heat the oil in a large nonstick skillet over medium-high heat. Season the salmon with 1/4 teaspoon each salt and pepper. Cook until opaque throughout, 3 to 4 minutes per side.
Serve the salmon with the creamed spinach and quinoa.