Preheat oven to 375 degrees F. Cut two 16-in-long pieces of parchment. Place parchment on a rimmed baking sheet. Cut ends off grapefruit so fruit stands on end. Take knife and carefully cut rest of peel and pith off grapefruit taking strips off vertically. Slice grapefruit into 1/4-inch thick rounds, then in half.
Stack ingredients in this order: spinach, grapefruit, tilapia, onions, garlic, sprinkle with salt and pepper.
Bring long ends together and fold three times so parchment is snug around the top of the fish. Tuck ends of parchment under the fish. Bake for 15 minutes, or until fish is cooked through (you'll have to open one packet to check).