Preheat the oven to 500 degrees F and place a pizza stone in the oven to heat. On a piece of parchment paper, roll out (or stretch) the pizza crust into a large 14-inch circle. Brush the pizza crust with 1 tablespoon olive oil and set aside.
Meanwhile, heat a skillet over medium heat, and add the butter, sliced crimini mushrooms and sliced shallots. Salt and pepper to taste. Then stir and saute for 10-12 minutes, until the onions and mushrooms are soft and slightly caramelized.
Re-stretch the pizza crust and spread a thin layer of spicy mustard over the top of the crust. Then spread the sauteed onions and mushrooms over the mustard. Place pieces of thinly sliced steak over the mushrooms, followed by a sprinkle of parmesan cheese.
Using a flat baking sheet or pizza paddle, slide the pizza onto the hot pizza stone and bake for 7-10 minutes until the edges are bubbled and crusty. Remove the pizza from the oven and sprinkle with fresh scallions before slicing and serving.