Preheat oven to 400°F. Coat a rimmed baking sheet with cooking spray.
Combine lemon zest, almonds, dill, 1 tablespoon oil, 1/2 teaspoon salt and pepper in a small bowl. Place fish on the prepared baking sheet and spread each portion with 1 teaspoon mustard. Divide the almond mixture among the portions, pressing it onto the mustard.
Bake the fish until opaque in the center, about 7 to 9 minutes, depending on thickness.
Meanwhile, heat the remaining 2 teaspoons oil in a Dutch oven over medium heat. Add garlic and cook, stirring, until fragrant but not brown, about 30 seconds. Stir in spinach, lemon juice and the remaining 1/2 teaspoon salt; season with pepper. Cook, stirring often, until the spinach is just wilted, 2 to 4 minutes. Cover to keep warm. Serve the fish with the spinach and lemon wedges, if desired.