Method:
Preheat oven to 400°F. Line a baking sheet with parchment paper and set aside.
Heat 1 tablespoon oil in a medium saucepan over medium-high heat. Add onion and cook, stirring often, until just softened, 2 to 3 minutes. Add quinoa, 2 cups water, and salt and bring to a boil. Reduce heat to medium-low, cover and simmer 15 minutes. Stir in spinach, cover and continue to cook until liquid is completely absorbed and quinoa is tender, about 3 minutes longer. Remove from heat and stir in tomatoes and pepper.
Meanwhile, brush halibut steaks with remaining 2 teaspoons oil and rub with Cajun seasoning to coat steaks evenly on all sides. Place on prepared baking sheet and bake until opaque throughout, about 20 minutes. Serve with quinoa pilaf.