Preheat the oven to 500 degrees . 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 to 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.