There’s no real substitute for the yellow aji peppers used commonly in Peru. They are quite hot and have a grassy flavour. I have used serrano peppers here, which is not quite the same but still makes this dish inspiring. Halibut is not a fish used in Peru. You would more likely find this made with Chilean sea bass. I recommend using sustainable fish if you can find it.
2 tbsp olive oil
1/4 cup chopped shallots
2 tsp seeded chopped serrano pepper
1 tsp grated lime zest
1 cup red quinoa, well rinsed
1 3/4 cups warm water
1 cup chopped fresh tomatoes or drained canned tomatoes
1/2 cup thinly sliced red onion
1/4 cup chopped mint
2 tbsp chopped cilantro
1 tsp chopped garlic
Salt and freshly ground pepper
4 cups baby spinach
4 6-oz (175-g) halibut fillets
Mint and cilantro leaves to garnish
Preheat oven to 400 F.
Heat oil in a wide ovenproof sauté pan over medium heat. Add shallots and sauté for 2 minutes or until softened. Add 1 tsp serrano pepper and lime zest and sauté 1 minute longer. Stir in quinoa and sauté for a minute or two or until slightly toasted. Add water and bring to boil. Reduce heat to low and simmer until water is absorbed, about 12 minutes. It will continue to cook in the oven. Season with salt.
Combine tomatoes, red onion, mint, cilantro, garlic and remaining 1 tsp serrano pepper. Stir 1/4 cup of mixture into quinoa. Lay spinach over quinoa and place fish on top. Season with salt and pepper. Spoon remaining tomato mixture evenly over fish.
Bake for 10 minutes covered. Uncover and bake for 5 minutes longer or until fish is white throughout but not flaky. Remove fish. Mix quinoa, spinach and tomato mixture to incorporate and spoon onto 4 plates. Lay fish on top of quinoa and garnish with mint and cilantro leaves.