Clean and finely chop the portobello mushrooms into very small pieces.
Heat oil and butter together in a frying pan over medium heat, and when it's hot, add the mushrooms with a large pinch of salt.
Cook the mushrooms, stirring frequently, until they darken and reduce in size. Keep cooking for 5-10 minutes on low heat to help them evaporate as much of their moisture content as possible. Remove from the heat and allow to cool completely.
Make the stuffing mix
Once the mushrooms have cooled, place them into a large mixing bowl.
Grate the onion and garlic into the bowl with the cooked mushrooms. Then add the egg, fresh parsley, dried thyme, lemon zest, lemon juice, and breadcrumbs.
Use a large spoon to thoroughly mix everything together, and season with salt and pepper.
Bake the stuffing
Preheat the oven to 150℃/300℉ and grease an oven dish with butter.
Portion the mushroom stuffing into six even-sized balls by rolling/squeezing it in your hands. Place the stuffing balls into the dish and add a small piece of butter on top of each one. Sprinkle with salt and pepper.
Bake the walnut and portobello mushroom stuffing balls for one hour.
Notes
I always recommend panko breadcrumbs as they help keep the stuffing balls moist. If you can't find them, use a packet of store-bought salad croutons and roughly crush them up with a rolling pin. Don't have the oven too high, it needs to be low, or the stuffing balls will dry out. You can make the stuffing balls in advance and reheat them, or even freeze them. Wrap them in plastic wrap and aluminum foil or place them in an airtight container.