Preheat the oven to 350°F. Line a large baking sheet with parchment paper.
Process the mushrooms: Add the mushrooms to a food processor and pulse until they are minced (stop while they still have some texture—the goal is not to liquefy them). Remove from the food processor and set aside.
Process the oats: Add the oats to the food processor (no need to clean the food processor first). Pulse until the oats are coarsely broken down.
Cook the shallot: Heat a large sauté pan over medium heat. Add the shallot and a splash of water. Cook for 3 to 4 minutes or until soft, adding more water as needed to prevent sticking.
Cook the mushrooms: Add the mushrooms and cook, stirring occasionally, for 6 to 8 minutes, or until the mushrooms are browned and most of the liquid has evaporated.
Cook the celery, tamari, and miso: Stir in the celery, tamari, and miso. Cook for an additional 3 to 4 minutes.
Add the remaining ingredients: Add the vegetable broth to the pulsed oats and stir to combine. Then, add the oat mixture to the pan, along with the rice, lentils, nutritional yeast, garlic powder, rosemary, sage, thyme, and salt (if using). Stir well and remove from heat.
Roll: Roll the mixture into 12 large balls and place them on the prepared baking sheet.
Bake: Bake for 30 to 35 minutes, or until the edges are starting to brown. Serve warm.