Step 1

Prepare grill for medium-high heat or preheat oven to 475°. In a large skillet, cook spinach in butter and 1 teaspoon oil over medium-high heat until wilted, 8-10 minutes.

Step 2

Divide spinach among four 18x12-in. pieces of heavy-duty nonstick foil, placing food on dull side of foil. Place tilapia on top of spinach; sprinkle with salt and pepper. Top with onion, lemon juice, garlic and remaining 2 teaspoons olive oil. Fold foil around mixture, sealing tightly.

Step 3

Place packets on grill or on a baking pan in oven. Cook until fish just begins to flake easily with a fork, 12-15 minutes. Open packets carefully to allow steam to escape. Serve with lemon wedges if desired.