Rinse to clean fish and pat fish dry. Season each side with course sea salt and pepper. In a small bowl, mix together juice from 1/2 of the lime, chili powder, & garlic powder. Spread thin layer evenly on both sides of tilapia fillets. Place 1/2 cup of spinach leaves on 4 large rectangles of foil (one rectangle for each fillet). Add a small amount (1/4 tsp) of olive oil across bed of spinach. Place seasoned Tilapia fillets over spinach. Sprinkle on shallots and season with a little extra sea salt. Top with a squirt of juice from remaining lime (lime slice optional also). Fold the foil up, creating an airtight pouch. Grill each pouch over indirect high heat for 5 minutes, flipping once during cooking time. (If you don't have a grill, you can also cook pouches at 350 degrees in the oven for about 15-20 minutes).