Heat 1 tablespoon olive oil in a medium skillet over medium heat.
Add the sliced white onion and sliced green bell pepper; sauté, stirring occasionally, about 4–6 minutes until softened.
Add the sliced cooked sirloin steak to the skillet and cook about 1 minute just to warm. Transfer the steak and vegetables to a plate and set aside.
In a mixing bowl, beat the 8 eggs until blended. Stir in 1/2 teaspoon salt and 1/2 teaspoon black pepper.
Reduce heat to medium-low and return the skillet to the stove.
Pour 1/4 of the beaten eggs (about 2 eggs) into the skillet, tilting the pan so the eggs form an even round. Cook until the edges are set and the bottom is lightly golden, about 1–2 minutes.
Spoon about 1/4 of the reserved steak and vegetables onto one half of the omelette, top with one provolone cheese slice, then fold the other half of the omelette over the filling. Cook 1–2 minutes more until the cheese melts and the omelette is cooked through. Transfer to a plate.
Repeat steps 6–7 three more times with the remaining egg mixture, steak mixture, and provolone slices to make 4 omelettes total.
Top each omelette with some of the diced tomato (divide the diced tomato evenly among the omelettes) and serve.