Serve warm or at room temperature.
Preheat oven to 400°F. Coat a rimmed baking sheet with cooking spray. Combine lemon zest, almonds, dill, 2 teaspoons olive 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 vegetable stock in a Dutch oven over medium heat. Stir in spinach, lemon juice and season with salt and pepper. Add garlic and cook, stirring, until spinach is wilted, 2 to 4 minutes. Cover to keep warm. Serve the fish with the spinach and lemon wedges, if desired.