Heat a large heavy pot over medium heat. Once hot, add the oil and mushrooms. Cook for about eight minutes, stirring infrequently, until the mushrooms have browned slightly and any water has evaporated.
Add the wine and let it cook for 2-3 minutes, or until reduced by about half. Add the vegetable broth.
100 ml (⅓ cup) dry wine, 300 ml (1 ¼ cups) vegetable broth
Cover and simmer for about 20 minutes, reducing the heat if it's boiling too hard.
In a small bowl, mix the corn starch and water until no lumps are visible. Keep the stew on the heat and stir in the starch mixture, cooking for another couple of minutes. As it bubbles, the stew will thicken.
1 tablespoon corn starch, 1 tablespoon water
Remove from the heat and transfer to a casserole dish** (pictured is about 20cm x 30cm, the ikea one). Set aside to cool to room temperature. Depending on your home temperature, this might take a couple of hours - I always place it in an unheated landing or hallway where it's much colder. Once cool, start the biscuits.
Biscuit Topping
Preheat the oven to 190°C (375°F). Add the flour, baking powder, salt, and baking soda to a large bowl. Whisk to combine.
300 grams (2 cups) spelt flour, 1 tablespoon baking powder, 1 ½ teaspoons sea salt, ½ teaspoon baking soda
Add the coconut oil and use your hands or a pastry cutter to incorporate the oil until it's in small pieces, about pea-sized.
120 grams (½ cup) solid coconut oil
Add the milk and vinegar, and rosemary if using, then stir into a shaggy dough.
Scoop about 12 biscuits onto the cooled vegetable base, using a cookie/muffin scoop or soup spoon. Make sure they're evenly placed.
Bake the vegetable pot pie for 35-40 minutes, or until the base is bubbling and the biscuits are golden brown. Cool for about ten minutes before serving. Leftovers can be refrigerated for a couple of days and should be reheated in the oven.
Notes
* Like water, almost every non-dairy milk (barring canned coconut milk) will have the same gram and ml measurements, and 190ml is hard to measure in a liquid measuring cup.** If you used an oven-safe cast iron or enamel pot to cook the vegetables in, you could use that to bake the pot pie, but I found in testing that this crowded the biscuits too much and made them a little stodgy.