Preheat the oven to 375 degrees Fahrenheit.
Clean the portobello mushrooms by wiping them with a damp cloth.
Gently remove the stalk from the centre of the mushroom.
Use a spoon to scrape out the gills from each mushroom (reserve the gills to add to the stuffing mixture).
Use a basting brush to coat the tops and sides of the mushrooms with 1 tablespoon of extra virgin olive oil, making sure that all of the areas that won’t be stuffed are thoroughly coated with oil.
Add the gills from the mushrooms, garlic, basil, parsley, walnuts, gluten-free oat flour, 1/4 cup olive oil, salt, and pepper to a food processor and process with several quick on/off pulses until everything is thoroughly mixed (alternatively, you could finely chop all the ingredients by hand and stir in a mixing bowl. I just prefer the time saver of the food processor).
Spoon the stuffing mixture into the mushrooms, making sure they are full but not overflowing.
Bake at 375 degrees Fahrenheit for 40 minutes.
Makes 6 vegan stuffed portobello mushrooms.