Clean the mushrooms with a damp paper towel and carefully break off stems. Season with salt and pepper and place on a baking tray lined with baking paper.
For the stuffing, mix the ricotta, chives, Parmesan, lemon zest, breadcrumbs, walnuts and salt and pepper. Fill each mushroom cap with a generous amount of filling and then bake in oven for 15-20 minutes until golden brown.
Heat a frypan. Brush the medallions with olive oil and season with salt and pepper. Grill for 5-6 minutes each side for medium-rare. Let the meat stand for 5 minutes before serving.
Any large mushrooms can replace the Portobellos suggested.