Line a baking sheet with aluminum foil. Make sure you cover the surface evenly so the juice from the vegetables and fish won't spill out while cooking.
Chop the vegetables (onions cut julienne), cut potatoes in wedges, slice the carrots
Place the potatoes and carrots in the dish and fold the foil to cover.
Bake for 10 minutes then remove the dish, open the foil and add the rest of the ingredients.
Place fish in the middle of the sheet.
Top with all the other vegetables.
Drizzle some oil.
Add salt, pepper, lemon juice, lemon zest and garlic and gently mix with a spatula.
Wrap foil well, return the dish to the oven, turn the oven to 375 and bake for 15 more minutes.
Remove baking dish from the oven, open the foil to expose the fish and return to oven. Bake for additional 5 minutes or until the fish gets a light golden crust.
Test with a fork for doneness. The fish should be flaky and easily come apart.
Carefully remove the foil pouch from the baking sheet and let it cool for a few minutes.
Serve the baked fish in foil directly from the pouch or transfer to a serving dish.