Preheat the oven to 400°F (200°C). If you're using a cast iron skillet, place it in the oven while it preheats so it gets nice and hot.

Make the flax egg by stirring together the ground flaxseed and water in a small bowl. Let it sit for about 5 minutes, until it turns thick and gel-like.

In a large bowl, whisk together the cornmeal, flour, sugar, baking powder, and salt.

In a separate bowl, combine the plant milk and apple cider vinegar and let it sit for a minute so it curdles slightly (this creates a simple vegan buttermilk). Whisk in the oil and flax egg.

Pour the wet ingredients into the dry ingredients and stir just until combined — a few lumps are fine, but don't overmix, or the cornbread will turn out tough.

Grease an 8x8-inch baking pan, or carefully remove the hot skillet from the oven and add a little oil, swirling to coat the bottom and sides (it should sizzle slightly). Pour in the batter and smooth the top.

Bake for 20 to 25 minutes, until the top is golden and a toothpick inserted in the center comes out clean.

Let cool in the pan for 5 to 10 minutes before slicing and serving.