Pre-heat the oven to 500°. Place the potatoes on a baking sheet and toss with 3 tablespoons EVOO, the garlic, the rosemary and salt and pepper to taste. Roast until the potatoes are tender and golden at the edges, 20-25 minutes.
While the potatoes are working, in a large skillet, heat the remaining 1/4 cup EVOO, 4 turns of the pan, over medium-high heat. Combine the cornmeal, flour, cheese and lemon peel on a plate. Season the snapper with salt and pepper, then coat evenly with the cornmeal mixture. Fry in the skillet, turning once, until golden brown, about 7 minutes total.
In a medium bowl, toss together the tomatoes, parsley, scallions, lemon juice and a drizzle of EVOO; season with salt and pepper.
Top the fish with the sauce and serve the potatoes alongside.