Coat the bottom of a large pot with the oil and place it over medium heat. Give it a minute to heat up, then add the mushrooms. Cook the mushrooms while stirring occasionally. After a few minutes they'll begin to release their juices. Continue cooking and stirring until most of the liquid has dried up, about 10 minutes.
Stir in the onion. Cook the onion and mushrooms while continuing to stir occasionally, about 5 minutes, until the onion is very tender.
Add the garlic and sprinkle in the flour. Stir to coat the mushrooms in the flour, and continue to cook the mixture for about 1 minute, stirring constantly.
Begin adding the milk, stirring in a bit at a time until it has all been added. Stir in the soy sauce, maple syrup, liquid smoke, thyme, sage, black pepper, and nutmeg. Raise the heat and bring the mixture to a boil, then lower the heat and let it simmer for about 10 minutes, until the liquid has thickened and the mushrooms are very tender.
Remove the pot from heat. Taste-test the gravy and season it with additional salt to taste.
To serve, place the biscuits on plates, optionally splitting them open first, then ladle the gravy over the biscuits.