In a medium bowl, toss together zucchini and zest and juice from 1 lemon; season with coarse salt and ground pepper. Cover with plastic wrap and marinate at room temperature until zucchini is softened, about 30 minutes.
Heat broiler, with rack in top position. On a rimmed baking sheet, place 4 slices crusty bread, 1/2 inch thick, in a single layer. Drizzle extra-virgin olive oil over each slice. Broil until browned around edges and tops are toasted, 2 minutes, rotating sheet frequently. To serve, divide zucchini among toasts, cutting bruschetta in half if desired. Top each with 1 or 2 wide shavings Parmesan and a drizzle of oil.