Serve warm or at room temperature.
Blanch onions in large pot of boiling salted water 15 seconds. Using slotted spoon, transfer to large bowl of ice water to cool. Trim root end if necessary, leaving core intact. Peel onions. Heat oil and vegetable stock in large nonstick skillet over high heat. Add onions and sauté until onions have deep golden brown spots, about 6 minutes (Add more stock as needed). Add orange juice and vinegar; bring to boil, scraping up browned bits. Reduce heat to medium-low, cover, and simmer until onions are just tender when pierced with knife, about 10 minutes. (Start the fish while the onions are cooking.) Using slotted spoon, transfer onions to medium bowl. Boil juices in skillet until syrupy and reduced to 2/3 cup, about 3 minutes. Pour over onions. 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.