Carefully remove mushrooms stems, keeping the mushroom caps intact. Discard half of the stems, and finely mince the other half.
Add the minced mushroom stems to a skillet over medium heat, along with the shallot, garlic, spinach, salt, and pepper. Cook 5 minutes, until the mushrooms are soft and the spinach is warmed through.
Add the tofu and nutritional yeast and stir to combine.
Arrange the mushroom caps in a baking dish, cavity side up.
Use a cookie scoop or spoon to scoop a tablespoon or so of the spinach tofu mixture into each mushroom cap.
Sprinkle the stuffed mushrooms with the breadcrumbs.
Bake for 20-25 minutes, until the mushrooms are soft. Then, turn the oven to broil and cook 5 minutes more until the breadcrumbs are golden brown. Serve immediately.