Shred Mushrooms: Preheat oven to 400°F (204°C). Clean mushrooms with a damp paper towel. Using two forks, shred the stems and caps roughly into pieces. Set on a parchment paper-lined baking sheet.
Bake Mushrooms: Drizzle with oil and 1 tsp of the garlic powder. Toss around to evenly coat the mushrooms, then bake for 20 minutes, or until mushrooms are a bit crispy and brown on the edges.
Bake Everything: Reduce oven temperature to 350°F (175°C). In a medium bowl, combine cooked mushrooms with all remaining ingredients. Transfer to a small casserole dish (optionally topping with more cheddar cheese) and bake for 20-25 minutes, or until cheese is melted and bubbly.
Serve with tortilla chips, celery, or carrots.
Store in an airtight container in the fridge for up to 5 days.