Turn oven broiler onto high. In a mixing bowl, combine both types of mushrooms and season with salt, pepper and olive oil. Transfer to a baking tray and place under broiler for 2 minutes or until golden. Remove, turn oven down to 325F and cool.
Using a food processor, cream the feta, lemon juice and heavy cream together. Season with salt and pepper. Set aside.
To assemble the flatbread, spread the whipped feta onto the base of the flatbread. Spread the caramelized onions atop the feta, then spread the mushrooms evenly over top. Drizzle with Truffle Honey. Place flatbread back into oven and bake for 8-10 minutes. Garnish with fresh Arugula and serve.