Step 1Heat the oven to 350 degrees. Coat a 9-inch cast-iron skillet, or other heavy ovenproof skillet, with oil.
Step 2Whisk the cornmeal, flour, baking powder, baking soda and salt in a large bowl. Whisk the oat milk, sugar and oil in another bowl until the sugar dissolves and the mixture is emulsified. Whisk in the vinegar to blend, then pour into the dry ingredients and whisk gently until smooth. Pour the batter into the prepared skillet.
Step 3Bake until golden brown, about 35 minutes; a toothpick inserted in the center of the cornbread should come out clean. Cool in the skillet on a rack. Serve warm or at room temperature.
Sage cornbread: Sprinkle fresh sage leaves all over the top of the batter before baking.
Rosemary cornbread: Stir 1 teaspoon minced fresh rosemary into the batter and sprinkle rosemary needles on top of the batter before baking.