Preheat oven to 450°. Cut baguette into 24 (1/4-inch-thick) slices; arrange in a single layer on baking sheets, and bake until golden (4-5 minutes). Heat 1 tablespoon extra-virgin olive oil until very hot but not smoking. Add mushroom mix; cook until water releases and mushrooms brown 4-5 minutes. Transfer to a bowl; cool. Toss mushrooms with 1 tablespoon olive oil, balsamic vinegar, oregano leaves, kosher salt, and black pepper. Thinly shave pecorino cheese. Divide arugula among baguette slices; top each with 1 tablespoon mushrooms and some pecorino.