Preheat the oven to 425 F (220 C). Line a baking sheet with parchment paper.
In a small bowl or glass, mix ½ cup plant milk with 1 teaspoon apple cider vinegar. Set aside.
In a medium mixing bowl, combine 2 cups whole wheat flour, 1 cup all purpose flour, 2 teaspoons baking powder, ½ teaspoon baking soda, ½ teaspoon salt, ½ teaspoon dried rosemary, and ½ teaspon dried thyme.
Use a fork to mash the sweet potato into the dry ingredients.
Add the plant milk and vinegar. Continue adding plant milk until you have a dough that will hold together. Sweet potatoes can be very sticky, so adjust the milk and flour.
Place the dough on a lightly floured surface and roll it flat to around ¾ inch in thickness. Using a biscuit cutter or glass dipped in flour, press straight down without twisting and cut the biscuits. You should end up with 8-10 depending on the size of the cutter you are using (1 ½ inches in diameter is a good guide).
Place the biscuits on the baking sheet and bake until the tops are lightly brown, 9-11 minutes.
Serve with warm lentil gravy.