Preheat the Oven: Set your oven to 400°F (200°C). This gives it time to reach the right temperature while you prepare the filling and mushrooms.
Prepare the Mushrooms: Gently twist and remove the stems from each mushroom, setting aside half of the stems. Wipe the mushroom caps with a damp paper towel to clean them. Finely mince the reserved stems and discard the rest.
Sauté the Filling Base: In a medium skillet over medium heat, warm a splash of oil or a tablespoon of water. Add the minced mushroom stems, chopped shallot, minced garlic, and drained spinach. Sprinkle in the salt and pepper. Sauté for about 5 minutes, stirring often, until the vegetables are soft and fragrant.
Incorporate the Tofu and Seasoning: Add the silken tofu and nutritional yeast directly into the skillet. Stir well until the tofu breaks down and everything is evenly combined. You should have a creamy, well-blended mixture with flecks of spinach throughout.
Fill the Mushroom Caps: Place the cleaned mushroom caps, gill-side up, into a large baking dish or sheet pan lined with parchment. Using a spoon or small cookie scoop, fill each mushroom generously with the tofu-spinach mixture.
Top with Crunchy Breadcrumbs: Evenly sprinkle the gluten-free breadcrumbs over each filled mushroom. This adds a crisp topping during baking.
Bake to Perfection: Transfer the baking dish to the oven and roast the mushrooms for 20 to 25 minutes, until they appear softened and begin to release juices.
Broil for a Golden Finish: Switch your oven to the broil setting and broil the mushrooms for an additional 4–5 minutes, or until the breadcrumbs turn golden brown and slightly crisp on top. Keep a close eye during this step to avoid burning.
Serve Warm: Carefully remove from the oven and let cool for a few minutes. Serve immediately while warm and flavorful.