In a food processor, pulse the fish until coarsely ground. Transfer to a bowl; add half the basil, the breadcrumbs, onion, half the bell pepper, egg, garlic, zest and 2 tbsp. mayonnaise. Season with salt and pepper and mix well. Using moistened hands, form into eight 3-inch-wide, 1-inch-thick cakes. Transfer to a plate; refrigerate 10 minutes.
In a bowl, mix 1/2 cup mayonnaise with the remaining basil, 3 tsp. lemon juice and the capers. Season the tartar sauce.
In a large nonstick skillet, heat 3 tbsp. oil over medium. Working in batches, cook the fish cakes until golden and crispy, about 5 minutes per side.
In a bowl, toss the spinach, remaining bell pepper, 2 tsp. juice and 1 tbsp. oil. Serve with the fish cakes and tartar sauce.