In a high speed food processor, add garlic scapes, basil, walnuts, parmesan cheese, lemon juice and pulse until combined.
With the blender or food processor running, stream in the olive oil. Scrape down the sides of the blender and continue blending as needed until the olive oil is emulsified and the pesto looks uniform.
Season with salt and pepper, to taste.
Fill Bella mushrooms with pesto.