To make the gravy: In a medium pot, drizzle the olive oil and place the pot over medium-high heat. Add the onion and stir for a couple of minutes, then add the mushrooms. Stir frequently until the mushrooms are browned and shrunken.
In a cup, combine the vegetable stock, milk, sherry, tamari and tapioca. Whisk to combine. Pour into the mushrooms, and whisk as the sauce thickens, reducing the heat to medium. Whisk in the black pepper. Once the gravy is thickened and bubbling, take it off the heat. Keep warm, or transfer to a storage container to cool completely, then cover and refrigerate for up to 3 days.
To make the croquettes: Cook the wild rice by bringing 3 cups water to a boil in a small pot, then adding the wild rice. Cook for about 20 minutes for hand-harvested wild rice (40 minutes or more for cultivated wild rice). When tender, drain and let cool.
Cut the potato into chunks and place in a steamer. Steam until very tender and they can be easily pierced with a paring knife. Transfer to a large bowl and let cool. Mash the potatoes until smooth. Measure 3 cups.
In a sauté pan, drizzle the olive oil and place the pan over medium-high heat. Add the onions and stir until they start to sizzle, then reduce the heat to medium-low and cook, stirring occasionally, for 5 minutes. Add the sage and stir until the sage is fragrant and dark green, about 2 minutes. Transfer the contents of the pan to the bowl with the potatoes, scraping to get all the oil and herbs.
Add the almond milk, nutritional yeast, salt and pepper, and mash to mix. Add the wild rice and mix well.