Remove the stems from the whole mushrooms, there should be enough room for the stuffing.
Chop the stems and keep them aside, we will be using this while making the stuffing.
Preheat the oven to 400 F or 200 C.
For making the stuffing
Place a pan over medium heat.
Add the butter, when the butter melts, add the onions and saute the onions till translucent.
Add the garlic and all the other veggies: diced celery, diced green peppers, chopped mushroom stems, and saute.
When the veggies start to brown up, add dried oregano, parsley leaves, salt, and ground pepper.
Add the bread crumbs and parmesan cheese, mix everything well.
The stuffing is ready, let it cool down.
Place 1 tbsp of stuffing into the cap of the baked mushrooms.
Place the stuffed mushrooms on a baking tray, and bake for 20 minutes until the mushrooms turn soft and the stuffing turns golden brown in color.
This is a perfect appetizer while serving if you prefer you can drizzle some balsamic vinegar.
Recipe by Cooking with Thas - Wholesome Homemade Recipes at https://thasneen.com/cooking/stuffed-gourmet-mushrooms-with-a-savoury-cheesy-filling-perfect-appetizer/