Preheat the oven to 375°F (190°C).
In a skillet over medium heat, add olive oil, steak, salt, and pepper. Cook until steak is browned, about 5 minutes.
Add onion and bell pepper to the skillet, cooking until softened, about 5 minutes.
On a baking sheet, spread out tortilla chips and top with steak mixture. Sprinkle cheddar and mozzarella cheese evenly over the top.
Bake for 10-15 minutes, until cheese is melted and bubbly.
For the queso dip, melt butter in a saucepan over medium heat. Stir in flour until smooth. Gradually add milk, stirring until thickened. Add cheddar cheese, salt, and pepper, stirring until cheese is melted.
Serve nachos with warm queso dip on the side.