Set oven to 400F. Place cast-iron skillet in oven while preheating.
Mix 1 tbsp ground flaxseed with 3 tbsp water. Set aside 5 minutes until gel-like.
Combine soy milk and apple cider vinegar. Rest 5 minutes. Add oil and flax egg. Whisk to combine.
Whisk cornmeal, flour, sugar, baking powder, and salt in a large bowl until combined.
Pour wet ingredients into dry. Fold until just combined. Stop when no large dry streaks remain. Do not overmix.
Remove hot skillet from oven with oven mitts. Add 1 tsp oil, swirl to coat. Pour batter in immediately.
Bake 18-22 minutes until golden and toothpick comes out clean. Rest in pan 10 minutes before slicing.