Cook the Veggies: In a pan over medium heat, add oil and sauté the diced onion and sliced bell pepper. Season with salt and black pepper. Cook for about 6 to 8 minutes, until the onions are soft and translucent and the peppers are tender. Stir in the minced garlic and cook for 1 to 2 more minutes, then remove from the pan and set aside.
Cook the Steak: In the same pan, heat the butter and oil over medium to medium-high heat. Add the shaved steak and let it sear undisturbed for about 2 minutes to get some browning. Then season with salt, paprika, mustard powder, and black pepper. Stir in Worcestershire sauce and beef broth, and cook for another 3 to 5 minutes, until the liquid reduces and the steak is juicy and tender. Turn off the heat and stir the veggies back in.
Assemble the Tacos: On each tortilla, sprinkle some cheese on one half, add a scoop of the steak and veggie mixture, and top with a little more cheese. Fold over the tortilla to close.
Toast the Tacos: Heat a pan or griddle over medium heat with a light coating of oil. Toast each taco for about 2 to 3 minutes per side, or until golden, crispy, and the cheese is fully melted.
Enjoy warm and bonus points with ketchup!