Preheat oven to 400 degrees F.
Grease a baking sheet with cooking spray. Set aside.
Carefully, rinse the dirt off of the mushrooms. Do not soak mushrooms in water. Instead, wipe off the dirt with a damp paper towel or lightly rinse under cool water and dry them with a towel carefully.
Remove, but do not discard, the stems. Set aside. Place the mushroom caps on the baking sheet.
Finely chop the mushroom stems, onions and garlic. Roughly chop spinach. Chop sun dried tomatoes in small pieces.
In a medium skillet, melt butter and add chopped mushroom stems. Cook for about 5 minutes, until most of the moisture has been removed.
Add in the onions, garlic, and sun dried tomatoes. Cook for 2 minutes, until the onions have softened a bit.
Reduce heat to low.
Sprinkle chickpea flour across the skillet. Stir together.
Add in dairy free milk and spinach and cook until spinach is wilted, about 3 minutes. *Add another splash to dairy free milk if it appears to dry.
Add in red pepper flakes and vegan Parmesan.
Taste mixture and add the salt and pepper if desired.
Remove from heat. Let cool for 5 minutes.
Fill the mushrooms caps with the filling, but do not overfill. They will puff up a bit while cooking.
Bake until the tops are golden brown, 20-25 minutes.
Top with fresh parsley if desired.