In a small saucepan over medium heat, heat the olive oil. Add the onions, mushroom stems and garlic, and cook for 5 minutes or until fragrant and slightly reduced. Add the walnuts and thyme, and cook for 1 minute to toast the walnuts lightly. Remove from heat and set aside.

In a small bowl, combine the chickpeas, ¼ cup of parmesan, parsley, balsamic vinegar, salt and pepper and use a fork to combine and mash to form a paste-like texture. Transfer the walnut mixture on top of the chickpea mixture and stir to combine.

Carefully place the mushrooms on the hot tray, spray with cooking spray and season lightly with salt. Spoon the filling into the mushrooms, and top with the remaining parmesan cheese.

Bake the preheated oven until the mushrooms are cooked through and the cheese has melted, about 15 minutes. Serve warm.