Rinse and drain the quinoa, then cook in the 1-quart pot according to the package directions.
While the quinoa cooks, warm the olive oil in the nonstick skillet over medium heat. Add the red onion slices and sauté until they soften, about 5 minutes. Add the minced garlic and sauté for 1 more minute. Add the spinach and stir, then cover with the lid, cooking until the spinach wilts, about 3 to 5 minutes. Stir in the cooked quinoa, add a little sea salt to taste, then cover and keep warm.
Rub the harissa paste on both sides of each piece of salmon, and place on the prepared baking sheet. Sprinkle a little kosher salt on top of each, and bake in the preheated oven until the fish flakes easily, about 20 minutes.
While the fish bakes, stir together the chopped mango, 2 tablespoons chopped red onion, mint, and orange juice in the medium bowl and allow to rest.
Serve each piece of salmon with the mango salsa and spinach quinoa on the side.