Preheat then oven to 400deg F.
Combine the cornmeal, flour, 1 teaspoon of the salt, sugar, baking powder, and cayenne, if using, in a mixing bowl. Add the egg and milk and mix well, but do not beat. Fold in the corn and onions.
Heat the lard or shortening in a 10-inch cast-iron or other ovenproof skillet over medium-high heat until almost smoking. Pour in the batter and cook for 3 to 4 minutes, or until the edges begin to turn golden. Place the skillet in the oven and bake for 45 minutes, or until golden brown. Sprinkle on top with the remaining 1/2 teaspoon salt. Let cool for about 5 minutes.
Slice the wedges to serve. The corn bread can be stored, wrapped in plastic wrap or foil, for up to three days in the refrigerator. To reheat, sprinkle the bread with a little water and place in the oven.