Preheat the oven to 425 degrees.
In a saucepan on medium heat, melt the butter and whisk in the flour. Cook for 1 minute while stirring. Slowly whisk in the milk and stir in half of the cheese until it is thick and smooth. Set aside.
In the meantime, heat the oil in a nonstick pan on medium heat. Saute the mushrooms, onion, and peppers until they soften up, approximately 5 - 6 minutes. Then add in the chopped steak and cook for another 2 - 3 minutes. Set aside on a plate.
Lightly grease a baking sheet or whatever kind of flat oven safe baking pan you are using to cook the pizza with nonstick spray.
Clean and dry the counter well. (make sure your hands are dried well before working with the dough) Sprinkle the counter with flour, place the pre-made dough on top of the flour, coating each side so the dough doesnt stick.
Roll out the dough using a rolling pin to fit the dimensions of the pan you are using.
Begin rolling out the dough using a rolling pin to stretch the dough to fit the baking sheet or pan you are using. Make sure it is being stretched evenly and avoid creating thin spots. (if the dough starts to stick while stretching it, sprinkle flour to prevent it from sticking)
Gently place the dough on the baking sheet or pan.
Evenly ladle and spread a layer of cheese sauce on the dough, leaving about an inch of dough without sauce around the edge.
Evenly spread the onions, peppers, mushrooms, and steak on the pizza. Top with the remaining cheese.
Bake the pizza in the oven for 10 minutes, then rotate the pan and bake until the crust is golden brown and crispy and the cheese is melted, approximately 8 - 10 minutes more.
Let the pizza sit for 2 - 3 minutes before cutting.