Preheat the oven to 355°F (180°C).
Drain the soaked cashews and add them to a blender. Add the water, nutritional yeast, white vinegar, and salt.
Blend on high speed for 15-30 seconds or until smooth. You will end up with a liquid cream. Transfer to a small bowl and set aside.
Heat the butter in a non-stick skillet over medium heat. Once hot, add garlic and sauté for 1 minute.
Next, add the sliced mushrooms and sauté for 3-5 minutes or until cooked. Season with soy sauce, dried rosemary, and ground black pepper. Cook for one more minute.
Turn off the heat and add the cashew cream to the sautéed mushrooms. Stir to combine. Set aside.
Assemble: Lightly grease a 7.5-inch tart pan with oil or butter. Unroll your pie crust (or flatten it, if homemade), and press it into the tart pan. Trim off any excess dough using a pair of scissors or by running a rolling pin over the tart pan. Poke the bottom and sides of the crust with a fork.
Transfer the mushroom filling to the pie crust and spread it evenly.
Bake for 35-40 minutes or until the crust is golden brown. If using a transparent glass tart pan, you can take a look at the bottom of the pan to check if the crust is cooked.
Let cool for a few minutes before serving! You can serve it as is or with a green salad on the side.
This mushroom tart will keep for up to 4 days in the refrigerator.
