Preheat oven to 400°F. Divide mahi mahi into two equal portions; set aside. Lay out two sheets of aluminum foil, each about 12-14 inches long. In the middle of each sheet, place a large handful of baby spinach leaves. Place one fish fillet on each bed of spinach. Season generously with salt and pepper. Place a few slices of onion over the top of each fillet and scatter the cherry tomatoes on and around the fish. Place two sprigs of thyme over top. Squeeze lemon and drizzle olive oil over fish.
Fold the sides of the foil inwards, then fold the top and bottom pinching the edges closed to form individual packets. Place packets on a baking sheet; bake for 18 -22 minutes, until the fish is opaque. Carefully open the packets to avoid the steam and spilling the juices. Transfer to a plate and spoon some of the juices over top. Makes 2 servings.
Approximate Nutrient Analysis per serving (based on 1 tablespoon drizzled oil but not including salt to taste):
350 calories, 10 g fat, 1 g saturated fat, 100 mg cholesterol, 250 mg sodium, 18 g carbohydrate, 5 g fiber, 9 g sugar, 51 g protein