Preparation
1. Preheat oven to 400°F.
2. Put spinach, with water still clinging to its leaves, into a large pot. Cover; steam the spinach over medium-high heat, stirring occasionally, until just wilted, about 5 minutes. Drain; when cool enough to handle, press out excess liquid. Chop and place in a small bowl. Stir in garlic. Season with salt and pepper.
3. To make a packet, lay two 20-inch sheets of foil on top of each other (the double layers will help protect the contents from burning); generously coat the top piece with cooking spray. Place one-quarter of the spinach mixture in the center of the foil. Lay a portion of sole over the spinach and arrange tomato slices over the sole. Season with salt and pepper.
4. Bring the short ends of the foil together, leaving enough room in the packet for steam to gather and cook the food. Fold the foil over and pinch to seal. Pinch seams together along the sides. Make sure all the seams are tightly sealed to keep steam from escaping. Repeat with more foil, cooking spray and the remaining ingredients.
5. Place the packets on a baking sheet. Bake the packets until the fish is cooked through and the vegetables are just tender, 10 to 12 minutes. To serve, carefully open both ends of the packets and allow the steam to escape. Use a spatula to slide the contents onto plates.