Preheat oven to 400 degrees
Toss the potatoes in a large bowl. Pour 1 tablespoon olive oil over the potatoes and toss to coat. Add the rosemary and garlic and toss again to coat evenly. (Don't wash the mixing bowl - set aside)
Spread the potatoes in a single layer on a large baking sheet. Season with salt and pepper.
Bake the potatoes for 15 minutes, then remove from the oven and turn the potatoes. Bake another 15 minutes and turn again, move the potatoes to one side of the pan.
Add the carrots and onion to the mixing bowl and toss to coat in any remaining olive oil. Add 1 teaspoon of oil if needed to cover the vegetables. Sprinkle with salt and pepper. Add the carrots and onion to the baking sheet and return to the oven for 15 minutes.
Remove the vegetables from the oven and make a hole in the center. Gently toss the fish fillets in the mixing bowl, adding more olive oil if needed to coat. Place the lemon in a single layer in the center of the baking pan and put the fish on top, skin side down. Sprinkle with the blackened seasoning and bake for 15-20 minutes or until the fish flakes easily and is just cooked through.
Allow the dish to rest for 5 minutes before serving.