Heat 1 tablespoon of vegetable oil in a large skillet over medium heat. Add the thinly sliced onion, green bell pepper, and red bell pepper. Season with salt and pepper. Sauté for about 5-7 minutes, or until the vegetables are softened and slightly caramelized. Remove them from the skillet and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Increase the heat to medium-high and add the thinly sliced sirloin steak. Season it with salt, pepper, and Worcestershire sauce. Cook for about 3-4 minutes until the steak is browned and cooked to your liking. Remove from heat.
In a mixing bowl, combine the freshly grated provolone cheese, shredded cheddar cheese, cornstarch, milk, and a pinch of salt. Mix until combined. Pour this mixture into a saucepan over low heat. Stir constantly until the cheese has melted and the mixture is smooth and creamy.
Preheat your oven to 350°F (175°C). On a large baking sheet, spread out the tortilla chips in an even layer. Top the chips with the sautéed vegetables, cooked steak, and then pour the cheese sauce generously over everything.
Bake the nachos in the preheated oven for about 10-12 minutes, or until the cheese is bubbly and the edges of the chips are golden. Keep an eye on them to avoid burning!
Once baked, remove the nachos from the oven and let them cool slightly before serving. You can garnish them with your favorite toppings such as jalapeños, sour cream, or fresh cilantro for an added burst of flavor.