Heat 1 tablespoon of the olive oil in a large sauté pan over medium heat. Slice all of the mushrooms and throw them into the pan. Let them set for a good minute before stirring. Then, let them sit another full minute.
Stir the mushrooms. They should be slightly glistening at this point. Season the mushrooms with salt and pepper. Keep stirring. Add the sliced garlic to the pan and stir. Once the garlic is slightly softened and translucent, about 1 minute, transfer the mushroom mixture to a food processor.
Place the olives, balsamic vinegar, tomato paste, tamari, and remaining 2 tablespoons of olive oil into the food processor as well. Run the motor on high until you have a smooth paste. You will have to stop the motor and scrape down the sides of the bowl a couple times. Transfer the pâté to a serving dish. I like to garnish this with a nice drizzle of balsamic glaze/reduction and some herbs to break up the brown-ness :)
I used all cremini because it’s a pretty accessible and tasty variety. If you want to mix it up with some shiitakes or other type, feel free!