Preheat oven to 350°F. Line a baking sheet with parchment paper. Set aside.
Heat 1 tablespoon of oil in a large skillet set over medium-high heat. Add in the onion, peppers, and a generous pinch of salt and pepper. Cook until the vegetables soften and begin to brown, about 5-7 minutes. Remove from the pan and set aside.
2 tablespoons vegetable oil, 1 onion, 1 green bell pepper, 1 red bell pepper, Kosher salt and freshly ground black pepper
Add the remaining oil to the pan. Add the steak in a single layer, cooking in batches if needed. Let cook until one side of the steak has a nice sear, then stir and cook until it is browned all over, about 4 minutes total. Remove from the heat.
1 pound sirloin steak
Arrange the tortilla chips on a baking sheet.
17 ounces tortilla chips
Top with steak, peppers and onions, and grated provolone cheese.
1½ cups freshly grated provolone cheese
Bake until the cheese is melted, about 10 minutes.
While the nachos cook, make the cheese sauce. In a medium bowl, toss the cheddar cheese and cornstarch together until the cheese is evenly coated.
2 cups freshly shredded cheddar cheese, 1 tablespoon cornstarch
Heat the milk in a medium saucepan over medium heat. Stir in the cheese, a handful at a time, until each handful is completely melted. Repeat until all the cheese has been added. Stir in the Worcestershire sauce.
¾ cup milk, ½ teaspoon Worcestershire sauce
Pour the cheese sauce over the nachos and serve immediately.