In a bowl, combine the ground chia seeds and water to make the chia egg. Mix to combine and let thicken at least 10 minutes.
In a food processor, pulse the walnuts, Brazil nuts, and cashews into a nutmeal like substance. Set aside.
Preheat the oven to 350°F. Line a 9-inch loaf pan with parchment paper with overhang on both of the long sides.
Bring a large deep skillet to medium-high heat, add the olive oil, onion, garlic, and mushrooms. Cook, stirring often, until softened, about 10 minutes. Add the sage, nutritional yeast, tamari, tahini, rosemary, thyme, oregano, and salt. Mix to combine.
Add the mushroom mixture to the food processor with the nuts and pulse a few times until the mixture is well combined, ground, and slightly doughy in texture.
Transfer the mixture back to the skillet (with the heat off) and add the chia egg and cooked brown rice. Mix everything to combine. Scoop the nut roast mixture into the prepared loaf pan and press gently to create a smooth, even surface.
Bake the nut roast in the oven for 50 minutes, or until lightly golden around the edges and on the top. Let cool slightly in the pan, about 10 minutes, to solidify before serving.
To make the mushroom topping (optional): Melt butter in a skillet on medium-high heat and add the cremini mushrooms. Cook, stirring often, until the mushrooms have browned and turned golden in color, about 12 minutes.
To make the vegan gravy: In a small saucepan, melt the butter on medium heat, add the vegetable broth, miso, onion powder, tamari, and nutritional yeast. Whisk to combine. Add the cornstarch and whisk again. Simmer on low heat until the gravy has thickened, about 8-10 minutes.
Serve the nut roast on a platter topped with the mushroom topping and vegan gravy on the side.
Notes
Mushroom topping and vegan gravy can be made while the nut roast is cooking in the oven.
Gravy can be made up to 4 days in advance. Reheat in a saucepan on medium-low heat. Add splashes of water or vegetable broth as needed to thin.