Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper.
In a large mixing bowl, combine the diced gyro meat, diced tomatoes, diced red onions, tzatziki sauce, eggs, feta cheese, oregano, breadcrumbs, salt, and pepper. Mix until well combined.
Remove the stems of the mushrooms and gently wipe the caps clean with a damp cloth.
Spoon the gyro mixture into the cavity of each mushroom, making sure to fill each one to the top.
Place the stuffed mushrooms on the prepared baking sheet and bake for 20-25 minutes, or until the tops are golden brown and the mushrooms are tender.
Serve hot, garnished with extra feta cheese and a drizzle of tzatziki sauce, if desired.
