Preheat the oven to 400°F degrees. In a bowl, toss together the mushrooms, oil and sea salt and mix well. Lay the mushrooms evenly over a lined baking sheet. Roast for 20 minutes until slightly browned. Flip halfway. Set the cooked mushrooms aside while you prepare the flatbread.
To make the Romesco sauce, blend together all the ingredients in a food processor or blender until smooth, yet slightly chunky. Set aside.
Warm the flatbread in the oven for a few minutes on 350°F. To assemble, layer a generous helping of the Romesco sauce over the entire flatbread. Then add the roasted mushrooms, and top with the micro-greens and chopped hazelnuts. Enjoy!