Place the steak in the freezer about 30 minutes before it is needed.
Melt butter in a large skillet over medium-high heat. Add onion, season with salt, and saute, stirring frequently, for 4 minutes. Add 1 tablespoon oil to the skillet along with bell pepper and mushrooms. Saute until mushrooms and bell pepper are tender, about 8 minutes. Remove vegetables from the skillet and set aside. Clean the skillet and allow it to cool down.
Preheat the oven to 400 degrees F (200 degrees C).
Remove steak from the freezer and slice into very thin slices. Season steak with freshly cracked salt, pepper, and garlic powder.
Add remaining 2 tablespoons oil to the skillet and heat over medium-high heat. Drop seasoned steak pieces into the hot oil and sauté until the meat is no longer pink, about 5 minutes. Remove steak from the skillet and drain any drippings.
Place 2 pizza crusts on pieces of foil or baking sheets, depending on their weight. Spread about 1/4 cup sour cream over each crust. Sprinkle each with 1 teaspoon minced garlic and 2 tablespoons Parmesan cheese. Top each crust evenly with steak, followed by vegetable mixture. Top pizzas with Provolone cheese pieces.
Bake in the preheated oven until the cheese has melted nicely and the edges of the crust are brown, about 10 minutes.