Preheat oven to 375 degrees. Lay a large piece of heavy-duty foil on a baking sheet, and spray with nonstick spray.
Place spinach, tomatoes, and corn on the center of the foil. Sprinkle with oregano and onion powder.
In a medium bowl, combine lemon juice, oil, chopped garlic, salt, and pepper. Whisk until uniform. Add shrimp, and toss to coat. Place over veggies.
Cover with another large piece of foil. Fold together and seal all four edges of the foil pieces, forming a well-sealed packet.
Bake for 20 minutes, or until shrimp have cooked through and veggies have softened.
Cut packet to release steam before open entirely.