Make the Sourdough dough at least one day in advance. Dust your countertop with a good handful of flour. Remove dough from fridge and shape into a ball and cut it into 3 pieces of approximately 230 g each. Form each one into a ball. Cover with plastic wrap and let rest for 60 – 90 minutes.
Prepare the Marinara Sauce. In a bowl, mix together canned Italian Tomatoes, Tomato Paste, Olive Oil, Oregano, Basil, Garlic, Sichuan Peppercorn, Salt and Ground Black Pepper. Refrigerate. You can make this a day in advance as well.
In a bowl, season the Steak with Salt and Ground Black Pepper. Set aside.
In a frying pan, add the Unsalted Butter on medium-high heat. Once melted, add the sliced Red Onion and Balsamic Vinegar. Stir for a few minutes until the Red Onion caramelizes. Remove and set aside.
Remove Marinara Sauce from the fridge. Mix in Balsamic Vinegar and Worcestershire Sauce. Set aside.
In a frying pan, add 1-2 tbsp of Olive Oil on high heat. Sear the Steak until rare or medium-rare. Set aside.
Form the Sourdough crust into a 10-inch round (or use frozen crust).
Spread the Sauce on the crust and top evenly with Red Onions, Buffalo Mozzarella and Gruyere Cheese.
Bake on a pizza stone in an oven preheated for 45-60 minutes at 450 F / 250 C. Baking time is 10-12 minutes.
Meanwhile, slice the Steak into thin strips or bite-size pieces.
Remove pizza from oven. Evenly spread Steak and Spinach on top and return to oven for another 1-2 minutes.
Remove pizza from oven. Drizzle with Steak Sauce and sprinkle with shavings of Parmesan and diced Green Onions.