In a large cast-iron skillet or saucepan, heat 1 tablespoon of oil over medium heat. Add the chopped onions and garlic. Cook for 5 minutes or until the onions are fragrant.
Add the celery and carrots and cook for another 5 minutes. Sprinkle the flour, salt, thyme and summer savory. Sauté for 1-2 minutes. Add the potatoes, then slowly add the broth and plant-based milk, stirring as you go.
Bring the mixture to a boil, then reduce to low-medium heat and let it simmer, uncovered for 10-15 minutes or until the vegetables are fork-tender. Add the canned lentils in the last 5 minutes of cooking.
Meanwhile, prepare the biscuits: Combine the milk and apple cider vinegar to make your vegan buttermilk. Set aside. Add the flour, baking powder and salt to a large bowl. Whisk until combined. Add the cold vegan butter to the dry ingredients and cut into the flour using a pastry cutter or fork until you get a coarse crumb. Add the prepared buttermilk, then mix everything together. Don’t overwork the dough! It should look crumbly. Place the dough on a lightly floured surface. Lightly press the dough to form a rectangle using your hands or a rolling pin. Fold the dough, one side to the center, then the other. Flatten, then repeat this step 2 more times. This helps you get flaky biscuits. Roll out the dough using a rolling pin until about ½ inch thick. Cut to make about 12 biscuits. Make sure not to twist the cutter as this will seal the sides and prevent the biscuits from rising.
Once the vegetable mixture is done, remove it from the heat. If it's not already in a cast-iron skillet or oven-safe casserole, transfer it. Top the vegetables evenly with the biscuits. You might have some extra biscuits, which you can bake later. Brush the biscuits with a small amount of plant-based milk.
Bake for 15 minutes at 425°F or until the biscuits are baked and lightly golden. Serve immediately.